Analytical Geometry, 1/e: 2D and 3D 9788131773604, 9789332517639, 8131773604, 9332517630

Table of contents :
Cover......Page 1
Dedication......Page 6
Brief Contents......Page 8
Contents......Page 10
About the Author......Page 20
Preface......Page 22
1.1 Introduction......Page 24
1.1.1 Distance between Two Given Points......Page 25
1.2.1 Coordinates of the Point that Divides the Line Joining Two Given Points in a Given Ratio......Page 26
1.2.3 Centroid of a Triangle Given its Vertices......Page 27
1.2.4 A rea of Triangle ABC with Vertices A(x1, y1), B(x2, y2) and C(x3, y3)......Page 28
1.2.5 Area of the Quadrilateral Given its Vertices......Page 29
Illustrative Examples......Page 30
Exercises......Page 50
2.1.1 Determination of the General Equation of a Straight Line......Page 54
2.1.2 Equation of a Straight Line Parallel to y-axis and at a Distance of h units from x-axis......Page 55
2.3 Slope-intercept Form of a Straight Line......Page 56
2.4 Intercept Form......Page 57
2.7 Normal Form......Page 58
2.8 Parametric Form and Distance Form......Page 59
2.9 Perpendicular Distance on a Straight Line......Page 60
2.10 Intersection of Two Straight Lines......Page 61
2.11 Concurrent Straight Lines......Page 63
2.12 Angle between Two Straight Lines......Page 64
Illustrative Examples......Page 65
Exercises......Page 98
3.1 Introduction......Page 106
3.3 Angle between the Lines Represented by ax 2 + 2hxy + by 2 = 0......Page 107
3.4 Equation for the Bisector of the Angles between the Lines Given by ax 2 + 2hxy + by 2 = 0......Page 108
3.5 Condition for General Equation of a Second Degree Equation to Represent a Pair of Straight Lines......Page 110
Illustrative Examples......Page 114
Exercises......Page 140
4.3 Centre and Radius of a Circle Represented by the Equation x 2 + y 2 + 2gx + 2fy + c = 0......Page 142
4.4 Length of Tangent from Point P(x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0......Page 143
4.6 Equation of Circle with the Line Joining Points A (x1, y1) and B (x2, y2) as the ends of Diameter......Page 144
4.7 Condition for the Straight Line y = mx + c to be a Tangent to the Circle x 2 + y 2 = a 2......Page 145
4.8 Equation of the Chord of Contact of Tangents from (x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0......Page 146
4.9 Two Tangents can Always be Drawn from a Given Point to a Circle and the Locus of the Point of Intersection of Perpendicular Tangents is a Circle......Page 147
4.10.1 Polar of the Point P (x1, y1) with Respect to the Circle x 2 + y 2 + 2gx + 2fy + c = 0......Page 148
4.11.1 Condition for the Lines lx + my + n = 0 and l1x + m1y + n1 = 0 to be Conjugate Lines with Respect to the Circle x 2 + y 2 = a2......Page 149
4.12 Equation of a Chord of Circle x 2 + y 2 + 2gx + 2fy + c = 0 in Terms of its Middle Point......Page 150
4.13 Combined Equation of a Pair of Tangents from (x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0......Page 151
Illustrative Examples......Page 152
Exercises......Page 187
5.1 Radical Axis of Two Circles......Page 198
5.2 Orthogonal Circles......Page 199
5.3 Coaxal System......Page 200
5.4 Limiting Points......Page 201
5.5 Examples (Radical Axis)......Page 203
5.6 Examples (Limiting Points)......Page 215
Exercises......Page 223
6.2 General Equation of a Conic......Page 226
6.3 Equation of a Parabola......Page 227
6.5 Different Forms of Parabola......Page 228
Illustrative Examples Based on Focus Directrix Property......Page 230
6.6 Condition for Tangency......Page 233
6.8 Perpendicular Tangents......Page 234
6.9 Equation of Tangent......Page 235
6.10 Equation of Normal......Page 236
6.11 Equation of Chord of Contact......Page 237
6.13 Conjugate Lines......Page 238
6.14 Pair of Tangents......Page 239
6.15 Chord Interms of Mid-point......Page 240
6.17 Chord Joining Two Points......Page 241
6.19 Point of Intersection of Tangents......Page 242
6.21 Number of Normals from a Point......Page 243
6.22 Intersection of a Parabola and a Circle......Page 244
Illustrative Examples Based on Tangents and Normals......Page 245
Illustrative Examples Based on Parameters......Page 266
Exercises......Page 287
7.2 Standard Equation of an Ellipse......Page 290
7.4 Position of a Point......Page 293
7.5 Auxiliary Circle......Page 294
Illustrative Examples Based on Focus-directrix Property......Page 295
7.6 Condition for Tangency......Page 300
7.7 Director Circle of an Ellipse......Page 301
7.8 Equation of the Tangent......Page 302
7.9 Equation of Tangent and Normal......Page 303
7.10 Equation to the Chord of Contact......Page 305
7.11 Equation of the Polar......Page 306
7.12 Condition for Conjugate Lines......Page 307
Illustrative Examples Based on Tangents, Normals, Pole-polar and Chord......Page 308
7.14 Equation of the Chord Joining the Points......Page 334
7.15 Equation of Tangent at ‘p ’ on the Ellipse......Page 336
7.16 Conormal Points......Page 337
7.17 Concyclic Points......Page 338
7.18 Equation of a Chord in Terms of its Middle Point......Page 340
7.19 Combined Equation of Pair of Tangents......Page 341
7.20 Conjugate Diameters......Page 343
7.20.1 Locus of Midpoint......Page 369
7.20.2 Property: The Eccentric Angles of the Extremities of a Pair of Semi-conjugate Diameter Differ by a Right Angle......Page 370
7.20.4 Property: The Tangents at the Extremities of a Pair of Conjugate Diameters of an Ellipse Encloses a Parallelogram Whose Area Is Constant......Page 371
7.20.5 Property: The Product of the Focal Distances of a Point on an Ellipse Is Equal to the Square of the Semi-diameter Which Is Conjugate to the Diameter Through the Point......Page 372
7.21.1 Property: Equi-conjugate Diameters of an Ellipse Lie along the Diagonals of the Rectangle Formed by the Tangent at the Ends of its Axes......Page 373
Illustrative Examples Based on Conjugate Diameters......Page 374
Exercises......Page 389
8.2 Standard Equation......Page 394
8.5 Rectangular Hyperbola......Page 396
8.6 Conjugate Hyperbola......Page 397
8.7.1 Equations of Asymptotes of the Hyperbola......Page 421
8.7.2 Angle between the Asymptotes......Page 423
8.8 Conjugate Diameters......Page 428
8.9.1 Equation of Rectangular Hyperbola with Reference to Asymptotes as Axes......Page 434
8.9.2 Equations of Tangent and Normal at (x1, y1) on the Rectangular Hyperbola xy = c 2......Page 435
8.9.3 Equation of Tangent and Normal at ctct, °  on the Rectangular Hyperbola xy = c 2......Page 436
8.9.6 Results Concerning the Rectangular Hyperbola......Page 437
8.9.8 Concyclic Points on the Rectangular Hyperbola......Page 438
Exercises......Page 454
9.2 Definition of Polar Coordinates......Page 458
9.3 Relation between Cartesian Coordinates and Polar Coordinates......Page 460
9.5 Polar Equation of a Straight Line in Normal Form......Page 461
9.6.1 Polar Equation of a Circle......Page 466
9.6.2 Equation of the Chord of the Circle r = 2a cos p on the Line Joining the Points (r1, p1) and (r2, p2).......Page 468
9.6.3 Equation of the Normal at ` on the Circle r = 2` cosp......Page 469
9.6.4 Equation of the Circle on the Line Joining the Points (a, ` ) and (b, a ) as the ends of a Diameter......Page 470
9.7.1 Polar Equation of a Conic......Page 474
9.7.2 Equation to the Directrix Corresponding to the Pole......Page 477
9.7.3 Equation to the Directrix Corresponding to Focus Other than the Pole......Page 478
9.7.4 Equation of Chord Joining the Points whose Vectorial Angles are ` - a and ` + a on the Conic......Page 479
9.7.5 Tangent at the Point whose Vectorial Angle is ` on the Coniclr=1+ ecosp......Page 480
9.7.6 Equation of Normal at the Point whose Vectorial Angle is a on the Conic......Page 481
9.7.7 Asymptotes of the Conic islr=1+ecosp (e>1)......Page 482
9.7.8 Equation of Chord of Contact of Tangents from (r1,p1) to the Conic......Page 483
9.7.9 Equation of the Polar of any Point (r1,p1) with Respect to the Coniclr=1+ ecosp......Page 484
Exercises......Page 508
10.2 Shift of Origin without Changing the Direction of Axes......Page 510
10.3 Rotation of Axes without Changing the Origin......Page 511
10.5 Invariants......Page 512
10.6 Conditions for the General Equation of the Second Degree to Represent a Conic......Page 513
10.8 Equation of the Conic Referred to the Centre as Origin......Page 515
10.9 Length and Position of the Axes of the Central Conic whose Equation is ax 2 + 2hxy + by 2 = 1......Page 517
10.10 Axis and Vertex of the Parabola whose Equation is ax 2 + 2hxy + by 2 + 2gx + 2fy + c = 0......Page 519
Exercises......Page 525
11.1 Rectangular Coordinate Axes......Page 526
11.2 Formula for Distance between Two Points......Page 529
11.2.1 Section Formula......Page 530
11.3 Centroid of Triangle......Page 532
11.5 Direction Cosines......Page 533
11.5.1 Direction Ratios......Page 534
11.5.2 Projection of a Line......Page 535
11.5.4 Angle between Two Given Lines......Page 536
Illustrative Examples......Page 538
Exercises......Page 550
12.2 General Equation of a Plane......Page 554
12.4 Equation of a Plane in Intercept Form......Page 555
12.5 Equation of a Plane in Normal Form......Page 556
12.7 Perpendicular Distance From a Point on a Plane......Page 558
12.8 Plane Passing Through Three Given Points......Page 560
12.9 To Find the Ratio in which the Plane Joining the Points (x1, y1, z1) and (x2, y2, z2) is Divided by the Plane ax + by + cz + d = 0.......Page 561
12.10 Plane Passing Through the Intersection of Two Given Planes......Page 562
12.11 Equation of the Planes which Bisect the Angle between Two Given Planes......Page 563
12.12 Condition for the Homogenous Equation of the Second Degree to Represent a Pair of Planes......Page 564
Illustrative Examples......Page 565
Exercises......Page 586
13.2 Equation of a Straight Line in Symmetrical Form......Page 592
13.3 Equations of a Straight Line Passing Through the Two Given Points......Page 594
13.4 Equations of a Straight Line Determined by a Pair of Planes in Symmetrical Form......Page 595
13.5 Angle between a Plane and a Line......Page 596
13.7 Conditions for a Line to Lie on a Plane......Page 597
13.9 Coplanar Lines......Page 598
13.10.1 Length and Equations of the Line of the Shortest Distance......Page 600
13.10.2 Equation of the Line of SD......Page 603
13.11 Equations of Two Non-intersecting Lines......Page 604
13.12 Intersection of Three Planes......Page 605
13.13 Conditions for Three Given Planes to Form a Triangular Prism......Page 607
Illustrative Examples......Page 608
Illustrative Examples (Coplanar Lines and Shortest Distance)......Page 625
Exercises......Page 638
14.2 The Equation of a Sphere with Centre at (a, b, c ) and Radius r......Page 644
14.3 Equation of the Sphere on the Line Joining the Points (x 1, y 1, z 1) and (x 2, y 2, z 2) as Diameter......Page 645
14.5 Equation of the Tangent Plane at (x1, y1, z1) to the Sphere x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d = 0......Page 646
14.6 Section of a Sphere by a Plane......Page 647
14.8 Intersection of Two Spheres......Page 648
14.10 Condition for Orthogonality of Two Spheres......Page 649
14.11.1 O btain the Equations to the Radical Plane of Two Given Spheres......Page 650
14.11.2 Properties of Radical Plane......Page 651
14.12.1 General Equation to a System of Coaxal Spheres......Page 652
14.12.3 Limiting Points......Page 653
Illustrative Examples......Page 654
Exercises......Page 690
15.2 Equation of a Cone with a Given Vertex and a Given Guiding Curve......Page 696
15.3 Equation of a Cone with its Vertex at the Origin......Page 699
15.4 Condition for the General Equation of the Second Degree to Represent a Cone......Page 702
15.5 Right Circular Cone......Page 708
15.5.1 Equation of a R ight Circular Cone with V ertex V (`, a, f ), Axis VL with Direction Ratios l, m, n and Semi-vertical Angle p......Page 709
15.5.2 Enveloping Cone......Page 710
15.6 Tangent plane......Page 718
15.6.1 Condition for the Tangency of a Plane and a Cone......Page 720
15.7.1 Equation of the Reciprocal Cone......Page 721
15.7.2 Angle between Two Generating Lines in Which a Plane Cuts a Cone......Page 722
15.7.3 Condition for Mutually Perpendicular Generators of the Cone......Page 725
Exercises......Page 729
16.2 Equation of a Cylinder with a Given Generator and a Given Guiding Curve......Page 730
16.3 Enveloping Cylinder......Page 731
16.4 Right Circular Cylinder......Page 732
Illustrative Examples......Page 733
Exercises......Page 747
Index......Page 750

Citation preview

Analytical Geometry

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Vittal's

Analytical Geometry 2D and 3D

P. R. Vittal Visiting Professor Department of Statistics University of Madras Chennai

Chennai • Delhi

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Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd. Licensees of Pearson Education in South Asia No part of this eBook may be used or reproduced in any manner whatsoever without the publisher’s prior written consent. This eBook may or may not include all assets that were part of the print version. The publisher reserves the right to remove any material in this eBook at any time. ISBN 9788131773604 eISBN 9789332517639 Head Office: A-8(A), Sector 62, Knowledge Boulevard, 7th Floor, NOIDA 201 309, India Registered Office: 11 Local Shopping Centre, Panchsheel Park, New Delhi 110 017, India

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To my grandchildren Aarav and Advay

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Brief Contents About the Author Preface

xix xxi



1 Coordinate Geometry

1-1



2 The Straight Line

2-1



3 Pair of Straight Lines

3-1



4 Circle

4-1



5 System of Circles

5-1



6 Parabola

6-1



7 Ellipse

7-1



8 Hyperbola

8-1



9 Polar Coordinates

9-1



10 Tracing of Curves

10-1



11 Three Dimension

11-1



12 Plane

12-1



13 Straight Line

13-1



14 Sphere

14-1



15 Cone

15-1



16 Cylinder

16-1

Index

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I-1

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Contents About the Author Preface





1 Coordinate Geometry 1.1 Introduction 1.2 Section Formula Illustrative Examples Exercises

2 The Straight Line 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13

Introduction Slope of a Straight Line Slope-intercept Form of a Straight Line Intercept Form Slope-point Form Two Points Form Normal Form Parametric Form and Distance Form Perpendicular Distance on a Straight Line Intersection of Two Straight Lines Concurrent Straight Lines Angle between Two Straight Lines Equations of Bisectors of the Angle between Two Lines Illustrative Examples Exercises

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xix xxi

1-1 1-1 1-3 1-7 1-27

2-1 2-1 2-3 2-3 2-4 2-5 2-5 2-5 2-6 2-7 2-8 2-10 2-11 2-12 2-12 2-45

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Contents

3 Pair of Straight Lines 3.1 Introduction 3.2 Homogeneous Equation of Second Degree in x and y 3.3 Angle between the Lines Represented by ax 2 + 2hxy + by 2 = 0 3.4 Equation for the Bisector of the Angles between the Lines Given by ax 2 + 2hxy + by 2 = 0 3.5 Condition for General Equation of a Second Degree Equation to Represent a Pair of Straight Lines Illustrative Examples Exercises

4 Circle 4.1 Introduction 4.2 Equation of a Circle whose Centre is (h, k) and Radius r 4.3 Centre and Radius of a Circle Represented by the Equation x 2 + y 2 + 2gx + 2fy + c = 0 4.4 Length of Tangent from Point P(x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0 4.5 Equation of Tangent at (x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0 4.6 Equation of Circle with the Line Joining Points A (x1, y1) and B (x2, y2) as the ends of Diameter 4.7 Condition for the Straight Line y = mx + c to be a Tangent to the Circle x 2 + y 2 = a 2 4.8 Equation of the Chord of Contact of Tangents from (x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0 4.9 Two Tangents can Always be Drawn from a Given Point to a Circle and the Locus of the Point of Intersection of Perpendicular Tangents is a Circle

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3-1 3-1 3-2 3-2 3-3

3-5 3-9 3-35

4-1 4-1 4-1 4-1 4-2 4-3 4-3 4-4

4-5

4-6

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4.10 Pole and Polar 4.11 Conjugate Lines 4.12 Equation of a Chord of Circle x 2 + y 2 + 2gx + 2fy + c = 0 in Terms of its Middle Point 4.13 Combined Equation of a Pair of Tangents from (x1, y1) to the Circle x 2 + y 2 + 2gx + 2fy + c = 0 4.14 Parametric Form of a Circle Illustrative Examples Exercises

5 System of Circles 5.1 Radical Axis of Two Circles 5.2 Orthogonal Circles 5.3 Coaxal System 5.4 Limiting Points 5.5 Examples (Radical Axis) 5.6 Examples (Limiting Points) Exercises

6 Parabola 6.1 Introduction 6.2 General Equation of a Conic 6.3 Equation of a Parabola 6.4 Length of Latus Rectum 6.5 Different Forms of Parabola Illustrative Examples Based on Focus Directrix Property 6.6 Condition for Tangency 6.7 Number of Tangents 6.8 Perpendicular Tangents 6.9 Equation of Tangent 6.10 Equation of Normal

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4-7 4-8

4-9

4-10 4-11 4-11 4-46

5-1 5-1 5-2 5-3 5-4 5-6 5-18 5-26

6-1 6-1 6-1 6-2 6-3 6-3 6-5 6-8 6-9 6-9 6-10 6-11

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Contents

6.11 Equation of Chord of Contact 6.12 Polar of a Point 6.13 Conjugate Lines 6.14 Pair of Tangents 6.15 Chord Interms of Mid-point 6.16 Parametric Representation 6.17 Chord Joining Two Points 6.18 Equations of Tangent and Normal 6.19 Point of Intersection of Tangents 6.20 Point of Intersection of Normals 6.21 Number of Normals from a Point 6.22 Intersection of a Parabola and a Circle Illustrative Examples Based on Tangents and Normals Illustrative Examples Based on Parameters Exercises

7 Ellipse 7.1 Standard Equation 7.2 Standard Equation of an Ellipse 7.3 Focal Distance 7.4 Position of a Point 7.5 Auxiliary Circle Illustrative Examples Based on Focus-directrix Property 7.6 Condition for Tangency 7.7 Director Circle of an Ellipse 7.8 Equation of the Tangent 7.9 Equation of Tangent and Normal 7.10 Equation to the Chord of Contact 7.11 Equation of the Polar 7.12 Condition for Conjugate Lines Illustrative Examples Based on Tangents, Normals, Pole-polar and Chord

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6-12 6-13 6-13 6-14 6-15 6-16 6-16 6-17 6-17 6-18 6-18 6-19 6-20 6-41 6-62

7-1 7-1 7-1 7-4 7-4 7-5 7-6 7-11 7-12 7-13 7-14 7-16 7-17 7-18 7-19

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7.13 Eccentric Angle 7-45 7.14 Equation of the Chord Joining the Points 7-45 7.15 Equation of Tangent at ‘p ’ on the Ellipse x2 y2 7-47 + =1 a2 b2 7.16 Conormal Points 7-48 7.17 Concyclic Points 7-49 7.18 Equation of a Chord in Terms of its Middle Point 7-51 7.19 Combined Equation of Pair of Tangents 7-52 7.20 Conjugate Diameters 7-54 7.21 Equi-conjugate Diameters 7-84 Illustrative Examples Based on Conjugate Diameters 7-85 Exercises 7-100

8 Hyperbola 8.1 Definition 8.2 Standard Equation 8.3 Important Property of Hyperbola 8.4 Equation of Hyperbola in Parametric Form 8.5 Rectangular Hyperbola 8.6 Conjugate Hyperbola 8.7 Asymptotes 8.8 Conjugate Diameters 8.9 Rectangular Hyperbola Exercises

9 Polar Coordinates 9.1 Introduction 9.2 Definition of Polar Coordinates 9.3 Relation between Cartesian Coordinates and Polar Coordinates 9.4 Polar Equation of a Straight Line 9.5 Polar Equation of a Straight Line in Normal Form 9.6 Circle

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8-1 8-1 8-1 8-3 8-3 8-3 8-4 8-28 8-35 8-41 8-61

9-1 9-1 9-1 9-3 9-4 9-4 9-9

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Contents

9.7 Polar Equation of a Conic Exercises

10 Tracing of Curves 10.1 General Equation of the Second Degree and Tracing of a Conic 10.2 Shift of Origin without Changing the Direction of Axes 10.3 Rotation of Axes without Changing the Origin 10.4 Removal of XY-term 10.5 Invariants 10.6 Conditions for the General Equation of the Second Degree to Represent a Conic 10.7 Centre of the Conic Given by the General Equation of the Second Degree 10.8 Equation of the Conic Referred to the Centre as Origin 10.9 Length and Position of the Axes of the Central Conic whose Equation is ax 2 + 2hxy + by 2 = 1 10.10 Axis and Vertex of the Parabola whose Equation is ax 2 + 2hxy + by 2 + 2gx + 2fy + c = 0 Exercises

11 Three Dimension 11.1 Rectangular Coordinate Axes 11.2 Formula for Distance between Two Points 11.3 Centroid of Triangle 11.4 Centroid of Tetrahedron 11.5 Direction Cosines Illustrative Examples Exercises

12 Plane 12.1 Introduction 12.2 General Equation of a Plane

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9-17 9-51

10-1 10-1 10-1 10-2 10-3 10-3 10-4 10-6 10-6 10-8 10-10 10-16

11-1 11-1 11-4 11-7 11-8 11-8 11-13 11-25

12-1 12-1 12-1

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12.3 General Equation of a Plane Passing Through a Given Point 12.4 Equation of a Plane in Intercept Form 12.5 Equation of a Plane in Normal Form 12.6 Angle between Two Planes 12.7 Perpendicular Distance from a Point on a Plane 12.8 Plane Passing Through Three Given Points 12.9 To Find the Ratio in which the Plane Joining the Points (x1, y1, z1) and (x2, y2, z2) is Divided by the Plane ax + by + cz + d = 0. 12.10 Plane Passing Through the Intersection of Two Given Planes 12.11 Equation of the Planes which Bisect the Angle between Two Given Planes 12.12 Condition for the Homogenous Equation of the Second Degree to Represent a Pair of Planes Illustrative Examples Exercises

13 Straight Line

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12-2 12-2 12-3 12-5 12-5 12-7

12-8 12-9 12-10

12-11 12-12 12-33

13-1

13.1 Introduction 13-1 13.2 Equation of a Straight Line in Symmetrical Form 13-1 13.3 Equations of a Straight Line Passing Through the Two Given Points 13-3 13.4 Equations of a Straight Line Determined by a Pair of Planes in Symmetrical Form 13-4 13.5 Angle between a Plane and a Line 13-5 13.6 Condition for a Line to be Parallel to a Plane 13-6 13.7 Conditions for a Line to Lie on a Plane 13-6 13.8 To Find the Length of the Perpendicular from a Given Point on a Line 13-7 13.9 Coplanar Lines 13-7 13.10 Skew Lines 13-9

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13.11 Equations of Two Non-intersecting Lines 13.12 Intersection of Three Planes 13.13 Conditions for Three Given Planes to Form a Triangular Prism Illustrative Examples Illustrative Examples (Coplanar Lines and Shortest Distance) Exercises

14 Sphere

13-13 13-14 13-16 13-17 13-34 13-47

14-1

14.1 Definition of Sphere 14-1 14.2 The equation of a sphere with centre at (a, b, c ) and radius r 14-1 14.3 Equation of the Sphere on the Line Joining the Points (x 1, y 1, z 1) and (x 2, y 2, z 2) as Diameter 14-2 14.4 Length of the Tangent from P(x1, y1, z1) to the Sphere x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d = 0 14-3 14.5 Equation of the Tangent Plane at (x1, y1, z1) to the Sphere x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d = 0 14-3 14.6 Section of a Sphere by a Plane 14-4 14.7 Equation of a Circle 14-5 14.8 Intersection of Two Spheres 14-5 14.9 Equation of a Sphere Passing Through a Given Circle 14-6 14.10 Condition for Orthogonality of Two Spheres 14-6 14.11 Radical Plane 14-7 14.12 Coaxal System 14-9 Illustrative Examples 14-11 Exercises 14-50

15 Cone 15.1 Definition of Cone 15.2 Equation of a Cone with a Given Vertex and a Given Guiding Curve

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15-1 15-1 15-1

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15.3 Equation of a Cone with its Vertex at the Origin 15.4 Condition for the General Equation of the Second Degree to Represent a Cone 15.5 Right Circular Cone 15.6 Tangent Plane 15.7 Reciprocal Cone Exercises

16 Cylinder 16.1 Definition 16.2 Equation of a Cylinder with a Given Generator and a Given Guiding Curve 16.3 Enveloping Cylinder 16.4 Right Circular Cylinder Illustrative Examples Exercises

Index

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15-4 15-7 15-13 15-23 15-26 15-34

16-1 16-1 16-1 16-2 16-3 16-4 16-18 I-1

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About the Author P. R. Vittal was a postgraduate professor of Mathematics at Ramakrishna Mission Vivekananda College, Chennai, from where he retired as Principal in 1996. He was a visiting professor at Western Carolina University, USA, and has visited a number of universities in the USA and Canada in connection with his research work. He is, at present, a visiting professor at the Department of Statistics, University of Madras; Institute of Chartered Accountants of India, Chennai; The Institute of Technology and Management, Chennai; and National Management School, Chennai, besides being a research guide in Management Science at BITS, Ranchi. Professor Vittal has published 30 research papers in journals of national and international repute and guided a number of students to their M.Phil. and Ph.D. degrees. A fellow of Tamil Nadu Academy of Sciences, his research topics are probability, stochastic processes, operations research, differential equations and supply chain management. He has authored about 30 books in mathematics, statistics and operations research.

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Preface A successful course in analytical geometry must provide a foundation for future work in mathematics. Our teaching responsibilities are to instil certain technical competence in our students in this discipline of mathematics. A good textbook, as with a good teacher, should accomplish these aims. In this book, you will find a crisp, mathematically precise presentation that will allow you to easily understand and grasp the contents. This book contains both two-dimensional and three-dimensional analytical geometry. In some of the fundamental results, vector treatment is also given and therefrom the scalar form of the results has been deduced. The first 10 chapters deal with two-dimensional analytical geometry. In Chapter 1, all basic results are introduced. The concept of locus is well explained. Using this idea, in Chapter 2, different forms for the equation of a straight line are obtained; all the characteristics of a straight line are also discussed. Chapter 3 deals with the equation of a pair of straight lines and its properties. In Chapters 4 and 5, circle and system of circles, including coaxial system and limiting points of a coaxial system, are analysed. Chapters 6, 7 and 8 deal with the conic sections—parabola, ellipse and hyperbola. Apart from their properties such as focus and directrix, their parametric equations are also explained. Special properties such as conormal points of all conics are described in details. Conjugate diameters in ellipse and hyperbola and asymptotes of a hyperbola and rectangular hyperbola are also analysed with a number of examples. A general treatment of conics and tracing of conics is also provided. In Chapter 9, we describe polar coordinates, which are used to measure distances for some special purposes. Chapter 10 examines the conditions for the general equation of the second degree to represent the different types of conics. In Chapters 11 to 16, we study the three-dimensional analytical geometry. The basic concepts, such as directional cosines, are introduced in Chapter 11. In Chapter 12, all forms of plane are analysed with the help of examples. Chapter 13 introduces a straight line as an intersection of a pair of planes. Different forms of a straight line are studied; especially, coplanar lines and the shortest distance between two skew lines. Chapter 14 deals with spheres and

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xxii

Preface

system of spheres. In Chapters 15 and 16, two special types of conicoids— cone and cylinder—are discussed. A number of illustrative examples and exercises for practice are given in all these 16 chapters, to help the students understand the concepts in a better manner. I hope that this book will be very useful for undergraduate students and engineering students who need to study analytical geometry as part of their curriculum.

Acknowledgements I express my sincere thanks to Pearson Education, India, especially to K. Srinivas, Sojan, Charles, and Ramesh for their constant encouragement and for successfully bringing out this book.

P. R. Vittal

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Chapter

1

Coordinate Geometry

1.1  INTRODUCTION Let XOX′ and YOY′ be two fixed perpendicular lines in the plane of the paper. The line OX is called the axis of X and OY the axis of Y. OX and OY together are called the coordinate axes. The point O is called the origin of the coordinate axes. Let P be a point in this plane. Draw PM perpendicular to XOX′. The distance OM is called the x-coordinate or abscissa and the distance MP is called the y-coordinate or ordinate of the point P. y P

x′

O

x

M

X

y′

If OM = x and MP = y then (x, y) are called the coordinates of the point P. The coordinates of the origin O are (0, 0). The lines XOX′ and YOY′ divide the plane into four quadrants. They are XOY, YOX′, X′OY′ and Y′OX. The lengths measured in the directions OX and OY are considered positive and the lengths measured in the directions OX′ and OY′ are considered negative. The nature of the coordinates in the different quadrants is as follows: Quadrant

x-coordinate

y-coordinate

First

+

+

Second

-

+

Third

-

-

Fourth

+

-

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1-2

Analytical Geometry

The method of representing a point by means of coordinates was first introduced by Rena Descartes and hence this branch of mathematics is called the rectangular Cartesian coordinate system. Using this coordinate system, one can easily find the distance between two points in a plane, the coordinates of the point that divides a line segment in a given ratio, the centroid of a triangle, the area of a triangle and the locus of a point that moves according to a given geometrical law.

1.1.1  Distance between Two Given Points Let P and Q be two points with coordinates (x1, y1) and (x2, y2). Draw PL and QM perpendiculars to the x-axis, and draw QN perpendicular to PL. Then, y

P Q

O

M

N L

x



OL = x1, LP = y1, OM = x2, MQ = y2



QN = ML = OL - OM = x1 - x2



NP = LP - LN = LP - MQ = y1 - y2

In

DPQN, PQ2 = QN2 + NP2 = (x1 - x2)2 + (y1 - y2)2 ∴ PQ = ( x1 − x2 )2 + ( y1 − y2 )2



Note 1.1.1: The distance of P from the origin O is OP = x12 + y12 Example 1.1.1 If P is the point (4, 7) and Q is (2, 3), then

PQ = ( 4 − 2)2 + (7 − 3)2 = 4 + 16 = 20 = 2 5 units

Example 1.1.2 The distance between the points P(2, -5) and Q(-4, 7) is

PQ = (2 + 4)2 + ( −5 − 7)2 = 36 + 144 = 180 = 6 5 units

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1-3

Coordinate Geometry

1.2  SECTION FORMULA 1.2.1  Coordinates of the Point that Divides the Line Joining Two Given Points in a Given Ratio Let the two given points be P(x1, y1) and Q(x2, y2). Q (x2,y2)

m

y l

R

T

P (x1,y1) S O

L

N

M

x

Let the point R divide PQ internally in the ratio l:m. Draw PL, QM and RN perpendiculars to the x-axis. Draw PS perpendicular to RN and RT perpendicular to MQ. Let the coordinates of R be (x, y). R divides PQ internally in the ratio l:m. Then,

OL = x1, LP = y1, OM = x2, MQ = y2, ON = x, NR = y



PS = LN = ON - OL = x - x1; RT = NM = OM - ON = x2 - x



SR = NR - NS = NR - LP = y - y1; TQ = MQ - MT = y2 - y1

Triangles PSR and RTQ are similar. ∴

PS SR PR l = = = RT TQ RQ m x − x1 PS l l = ⇒ = ⇒ m( x − x1 ) = l ( x2 − x ) RT m x2 − x m

∴ x(l + m) = lx2 + mx1 Also

or x =

lx2 + mx1 l+m

y − y1 ly + my1 SR l l = ⇒ = ⇒y= 2 TQ m y2 − y m l+m

 lx + mx1 ly2 + my1  Hence, the coordinates of R are  2 , .  l+m l + m 

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1-4

Analytical Geometry

1.2.2  External Point of Division l P

Q

RÄ

m

If the point R′ divides PQ externally in the ratio l:m, then PR′ l PR′ l = ⇒ = R′ Q m QR′ − m Choosing m negative, we get the coordinates of R′. Therefore, the coordinates of R′ are  lx2 − mx1 , ly2 − my  .  l−m l−m  Note 1.2.2.1: If we take l = m = 1 in the internal point of division, we get the coordinates of the midpoint. Therefore, the coordinates of the midpoint of  x + x y + y2  PQ are  1 2 , 1 .  2 2 

1.2.3  Centroid of a Triangle Given its Vertices Let ABC be a triangle with vertices A(x1, y1), B(x2, y2) and C(x3, y3). A C′

2 G

B′

1 B

A′

C

Let AA′, BB′ and CC′ be the medians of the triangle. Then A′, B′, C′ are the midpoints of the sides BC, CA and AB, respectively. The coordinates of A′ are  x2 + x3 y2 + y3  . We know that the medians of a triangle are concurrent  2 , 2  at the point G called the centroid and G divides each median in the ratio 2:1. Considering the median AA′, the coordinates of G are   y2 + y3    x2 + x3   1⋅ x1 + 2 ⋅  2  1⋅ y1 + 2 ⋅  2     , 1+ 2 1+ 2      x + x + x3 y1 + y2 + y3  , (i.e.)  1 2   3 3

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Coordinate Geometry

1-5

1.2.4  Area of Triangle ABC with Vertices A(x1, y1), B(x2, y2)  and C(x3, y3) Let the vertices of triangle ABC be A(x1, y1), B(x2, y2) and C(x3, y3). C (x3,y3)

y

B (x2,y2)

A (x1,y1)

O

L

N

M

x

Draw AL, BM and CN perpendiculars to OX. Then, area D of triangle ABC is calculated as D = Area of trapezium ALNC + Area of trapezium CNMB - Area of trapezium ALMB 1 1 1 = (LA + NC ) ⋅ LN + ( NC + MB) ⋅ NM − ( LA + MB ) ⋅ LM 2 2 2 1 1 1 = ( y1 + y3 )( x3 − x1 ) + ( y3 + y2 )( x2 − x3 ) − ( y1 + y2 )( x2 − x1 ) 2 2 2 1 = [ x1 ( y1 + y2 − y1 − y3 ) + x2 ( y3 + y2 − y1 − y2 ) + x3 ( y1 + y3 − y3 − y2 )] 2 1 = [ x1 ( y2 − y3 ) + x2 ( y3 − y1 ) + x3 ( y1 − y2 )] 2 x1 y1 1 1 = x2 y2 1 2 x3 y3 1

Note 1.2.4.1: The area is positive or negative depending upon the order in which we take the points. Since scalar area is always taken to be a positive quantity, we take 1 ∆ = x1 ( y2 − y3 ) + x2 ( y3 − y1 ) + x3 ( y1 − y2 ) 2 Note 1.2.4.2: If the vertices of the triangle are (0, 0), (x1, y1) and (x2, y2), then ∆=

1 x1 y2 − x2 y1 . 2

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1-6

Analytical Geometry

Note 1.2.4.3: If the area of the triangle is zero, i.e. D = 0, then we note that the points are collinear. Hence, the condition for the points (x1, y1), (x2, y2) and (x3, y3) to be collinear is x1(y2 - y3) + x2(y3 - y1) + x3(y1 - y2) = 0



1.2.5  Area of the Quadrilateral Given its Vertices D

y

C

A

O

B

P

S

R

Q

x

Let ABCD be the quadrilateral with vertices A(x1, y1), B(x2, y2), C(x3, y3) and D(x4, y4). Draw AP, BQ, CR and DS perpendiculars to the x-axis. Then, Area of quadrilateral ABCD = Area of trapezium APSD + Area of trapezium DSRC + Area of trapezium CRQB - Area of trapezium APQB = 1 ( PA + SD ) PS + 1 ( SD + RC )SR + 1 ( RC + QB ) RQ − 1 ( PA + QB ) PQ 2 2 2 2 1 ( x4 − x1 )( y1 + y4 ) + ( x3 − x4 )( y3 + y4 ) + ( x2 − x3 )( y3 + y2 ) −  =   2 ( x2 − x1 )( y1 + y2 )  =

1 [( x1 y2 − x2 y1 ) + ( x2 y3 − x3 y2 ) + ( x3 y4 − x4 y3 ) + ( x4 y1 − x1 y4 )] 2

Note 1.2.5.1: This result can be extended to a polygon of n sides with vertices (x1, y1), (x2, y2)…… (xn, yn) as

Area =

1 [( x1 y2 − x2 y1 ) + ( x2 y3 − x3 y2 ) + …+ ( xn y1 − x1 yn )] 2

Locus When a point moves so as to satisfy some geometrical condition or conditions, the path traced out by the point is called the locus of the point. For example, if a point moves keeping a constant distance from a fixed point, the locus of the moving point is called circle and the fixed distance is called the radius of the circle. Moreover, if a point moves such that its

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Coordinate Geometry

1-7

distance from two fixed points are equal, then the locus of the point is the perpendicular bisector of the line joining the two fixed points. If A and B are p two fixed points and point P moves such that APB = then the locus of P is 2 a circle with AB as the diameter. It is possible to represent the locus of a point by means of an equation. Suppose a point P(x, y) moves such that its distance from two fixed points A(2, 3) and B(5, -3) are equal. Then the geometrical law is PA = PB ⇒ PA2 = PB2 ⇒ ( x − 2)2 + ( y − 3)2 = ( x − 5)2 + ( y + 3)2 ⇒ x 2 − 4 x + 4 + y 2 + 9 − 6 y = x 2 + 25 − 10 x + y 2 + 9 + 6 y ⇒ 6 x − 12 y − 21 = 0 ⇒ 2 x − 4 y − 7 = 0



Here, the locus of P is a straight line.

ILLUSTRaTIVE EXAMPLES Example 1.1 Find the distance between the points (4, 7) and (-2, 5).

Solution Let P and Q be the points (4, 7) and (-2, 5), respectively.

PQ 2 = ( x1 − x2 )2 + ( y1 − y2 )2 = ( 4 + 2)2 + (7 − 5)2 = 36 + 4 = 40 PQ = 40 units

Example 1.2 Prove that the points (4, 3), (7, -1) and (9, 3) are the vertices of an isosceles triangle.

Solution Let A(4, 3), B(7, -1), C(9, 3) be the three given points. Then AB2 = (4 - 7)2 + (3 + 1)2 = 9 + 16 = 25

BC2 = (7 - 9)2 + (-1 - 3)2 = 4 + 16 = 20



AC2 = (4 - 9)2 + (3 - 3)2 = 25 + 0 = 25



∴ AB = 5, BC = 20, AC = 5

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1-8

Analytical Geometry

Since the sum of two sides is greater than the third, the points form a triangle. Moreover, AB = AC = 5. Therefore, the triangle is an isosceles triangle.

Example 1.3 Show that the points (6, 6), (2, 3) and (4, 7) are the vertices of a right angled triangle.

Solution Let A, B, C be the points (6, 6), (2, 3) and (4, 7), respectively.

AB2 = (6 - 2)2 + (6 - 3)2 = 16 + 9 = 25



BC2 = (2 - 4)2 + (3 - 7)2 = 4 + 16 = 20



AC2 = (6 - 4)2 + (6 - 7)2 = 4 + 1 = 5



AC2 + BC2 = 20 + 5 = 25 = AB2

Hence, the points are the vertices of a right angled triangle.

Example 1.4 Show that the points (7, 9), (3, -7) and (-3, 3) are the vertices of a right angled isosceles triangle.

Solution Let A, B, C be the points (7, 9), (3, -7), (-3, 3), respectively.

AB2 = (7 - 3)2 + (9 + 7)2 = 16 + 256 = 272



BC2 = (3 + 3)2 + (-7 - 3)2 = 36 + 100 = 136



AC2 = (7 + 3)2 + (9 - 3)2 = 100 + 36 = 136



BC2 + AC2 = 136 + 136 = 272 = AB2

Hence, the points are vertices of a right angled triangle. Also, BC = AC. Therefore, it is a right angled isosceles triangle.

Example 1.5 Show that the points (4, -4), (-4, 4) and (4 3, 4 3) are the vertices of an equilateral triangle.

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Coordinate Geometry

1-9

Solution Let A, B, C be the points (4, -4), (-4, 4) and (4 3, 4 3), respectively.

AB2 = (4 + 4)2 + (-4 - 4)2 = 64 + 64 = 128 BC 2 = ( −4 − 4 3 )2 + ( 4 − 4 3 )2 = 16 + 48 + 32 3 + 16 + 48 − 32 3 = 128 AC 2 = ( 4 − 4 3 )2 + ( −4 − 4 3 )2 = 16 + 48 − 32 3 + 16 + 48 + 32 3 = 128



AB = BC = AC = 128

Hence, the points A, B and C are the vertices of an equilateral triangle.

Example 1.6 Show that the set of points (-2, -1), (1, 0), (4, 3) and (1, 2) are the vertices of a parallelogram.

Solution Let A, B, C, D be the points (-2, -1), (1, 0), (4, 3) and (1, 2), respectively. A quadrilateral is a parallelogram if the opposite sides are equal.

AB2 = (-2 - 1)2 + (-1 - 0)2 = 9 + 1 = 10



BC2 = (1 - 4)2 + (0 - 3)2 = 9 + 9 = 18



CD2 = (4 - 1)2 + (3 - 2)2 = 9 + 1 = 10



AD2 = (-2 - 1)2 + (-1 - 2)2 = 9 + 9 = 18



AB = CD = 10 BC = AD = 18

Since the opposite sides of the quadrilateral ABCD are equal, the four points form a parallelogram.

Example 1.7 Show that the points (2, -2), (8, 4), (5, 7) and (-1, 1) are the vertices of a rectangle taken in order.

Solution A quadrilateral in which the opposite sides are equal and the diagonals are equal is a rectangle. Let A(2, -2), B(8, 4), C(5, 7) and D(-1, 1) be the four given points.

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1-10

Analytical Geometry

AB2 = (2 - 8)2 + (-2 - 4)2 = 36 + 36 = 72 AC2 = (2 - 5)2 + (-2, -7)2 = 9 + 81 = 90 BC2 = (8 - 5)2 + (4 - 7)2 = 9 + 9 = 18

BD2 = (8 + 1)2 + (4 - 1)2 = 81 + 9 = 90

CD2 = (5 + 1)2 + (7 - 1)2 = 36 + 36 = 72

AD2 = (2 + 1)2 + (-2 - 1)2 = 9 + 9 = 18

AB = CD = 72

BC = AD = 18

AC = BD = 90

Thus, the opposite sides are equal and the diagonals are also equal. Hence, the four points form a rectangle.

Example 1.8 Prove that the points (3, 2), (5, 4), (3, 6) and (1, 4) taken in order form a square.

Solution A quadrilateral in which all sides are equal and diagonals are equal is a square. Let A, B, C, D be the points (3, 2), (5, 4), (3, 6), (1, 4), respectively. AB2 = (3 - 5)2 + (2 - 4)2 = 4 + 4 = 8

CD2 = (3 - 1)2 + (6 - 4)2 = 4 + 4 = 8

BC2 = (5 - 3)2 + (4 - 6)2 = 4 + 4 = 8;

 AD2 = (3 - 1)2 + (2 - 4)2 = 4 + 4 = 8

AC2 = (3 - 3)2 + (2 - 6)2 = 0 + 42 = 16

BD2 = (5 - 1)2 + (4 - 4)2 = 16 + 0 = 16

 AB = BC = CD = AD = 8

AC = BD =  16 = 4

Thus, all sides are equal and also the diagonals are equal. Hence, the four points form a square.

Example 1.9 Find the coordinates of the circumcentre of a triangle whose vertices are A(3, -2), B(4, 3) and C(-6, 5). Also, find the circumradius.

Solution A(3, −2)

S (x, y) B(4, 3)

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C(−6, 5)

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Coordinate Geometry

1-11

Let A(3, -2), B(4, 3), and C(-6, 5) be the given points. Let S(x, y) be the circumcentre of D ABC. Then SA = SB = SC = circumradius. Now

SA2 = (x - 3)2 + (y + 2)2 = x2 + y2 - 6x + 4y + 13



SB2 = (x - 4)2 + (y - 3)2 = x2 + y2 - 8x - 6y + 25



SC2 = (x + 6)2 + (y - 5)2 = x2 + y2 + 12x - 10y + 61



SA2 = SB2 ⇒2x + 10y = 12

(1.1)



SA2 = SC2 ⇒18x - 14y = -48

(1.2)

(1.1) × 9 ⇒ 18 x + 90 y = 108





(1.2) × 1 ⇒ 18 x − 14 y = −48 104 y = 156 156 78 3 = = y= 104 52 2

(1.1)  ⇒ 2 x + 10 ×

3 = 12 ⇒ 2 x = 12 − 15 = −3 2 ⇒x=−



3 2

 3 3 Hence, the circumcentre is  − ,  .  2 2 2

2

3  3 81 49 130  = Now SA2 =  3 +  +  −2 −  = +  2  2 4 4 4 Therefore, circumradius =

130 130 = units. 4 2

Example 1.10 Show that the points (3, 7), (6, 5) and (15, -1) lie on a straight line.

Solution Let A(3, 7), B(6, 5) and C(15, -1) be the three points. Then

AB2 = (3 - 6)2 + (7 - 5)2 = 9 + 4 = 13

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1-12

Analytical Geometry



BC2 = (6 - 15)2 + (5 + 1)2 = 81 + 36 = 117



AC2 = (3 - 15)2 + (7 + 1)2 = 144 + 64 = 208 ∴ AB = 13



BC = 117 = 9 × 13 = 3 13 AC = 208 = 16 × 13 = 4 13

AB + BC = AC = 4 13

Hence, the three given points lie on a straight line.

Example 1.11 Show that (4, 3) is the centre of the circle that passes through the points (9, 3), (7, -1) and (1, -1). Find its radius.

Solution Let A(9, 3), B(7, -1), C(1, -1) and P(4, 3) be the given points. Then

PA2 = (9 - 4)2 + (3 - 3)2 = 25 + 0 = 25



PB2 = (7 - 4)2 + (-1 - 3)2 = 9 + 16 = 25



PC2 = (1 - 4)2 + (-1 - 3)2 = 9 + 16 = 25



∴ PA = PB = PC = 25 = 5

Hence, P is the centre of the circle passing through the points A, B, C; its radius is 5.

Example 1.12 If O is the origin and the coordinates of A and B are (x1, y1) and (x2, y2), respectively, prove that OA⋅OB cosq = x1x2 + y1y2 where AOB = q .

Solution By cosine formula

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Coordinate Geometry

1-13

O(0, 0) q

A(x1, y1)

B(x2, y2)

2OA⋅OB cosq = OA2 + OB2 - AB2

= x12 + y12 + x22 + y22 - [(x1 - x2)2 + (y1 - y2)2]



= 2(x1x2 + y1y2)

Hence, OA⋅OB cosq = x1x2 + y1y2

Example 1.13 If tana, tanb and tang   be the roots of the equation x3 - 3ax2 + 3bx - c = 0 and the vertices of the triangle ABC are (tana, cota), (tanb, cotb ) and (tang, cotg ) show that the centroid of the triangle is (a, b).

Solution Given tana, tanb  and tang  are the roots of the equation

x3 - 3ax2 + 3bx - c = 0

(1.3)

Then

tana + tanb + tang  = 3a

(1.4)

tana tanb + tanb tang + tang  tana = 3b

(1.5)



tana tanb tang  = 1

Then dividing (1.5) by (1.6), cota + cotb + cotg = 3b

(1.6) (1.7)

tan a + tan b + tan g cot a + cot b + cot g  The centroid of D ABC is  ,   3 3 (i.e.) (a, b) from (1.4) and (1.7).

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1-14

Analytical Geometry

Example 1.14 If the vertices of a triangle have integral coordinates, prove that it cannot be an equilateral triangle.

Solution The area of the triangle with vertices (x1, y1), (x2, y2) and (x3, y3) is 1 ∆ = [ x1 ( y2 − y3 ) + x2 ( y3 − y1 ) + x3 ( y1 − y2 )] 2



(1.8)

Also, the area of D ABC is ∆=



3 2 a 4

(1.9)

where a is the side of the equilateral triangle. If the vertices of the triangle have integral coordinates, then D is a rational number. However, from (1.9) we infer that the area is 3 times a rational number. Hence, if the vertices of a triangle have integral coordinates, it cannot be equilateral.

Example 1.15 If t1, t2 and t3 are distinct, then show that the points (at12, 2at1), (at22, 2at2) and (at32, 2at3), a ≠ 0 cannot be collinear.

Solution x1 1 The area of the triangle ∆ = x2 2 x3 t12 = a2 t22 t32

y1 1 at12 1 2 y2 1 = at2 2 2 y3 1 at3

2at1 1 2at2 1 2at3 1

t1 1 t2 1 = a2 (t1 − t2 )(t2 − t3 )(t3 − t1 ) ≠ 0 t3 1

since t1, t2 and t3 are distinct. Hence, the three given points cannot be collinear.

Example 1.16 The vertices of a triangle ABC are (2, 3), (4, 7), (-5, 2). Find the length of the altitude through A.

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Coordinate Geometry

1-15

Solution The area of D ABC is given by A(2, 3)

L

B(4, 7)

C(−5, 2)

1 ∆ = [ x1 ( y2 − y3 ) + x2 ( y3 − y1 ) + x3 ( y1 − y2 )] 2 1 1 ∆ = [2(7 − 2) + 4(2 − 3) − 5(3 − 7)] = [10 − 4 + 20] = 13 square units 2 2 BC = ( 4 + 5)2 + (7 − 2)2 = 81 + 25 = 106 We know that

∆=

1 2 ∆ 2 × 13 26 106 BC ⋅ AL ⇒ AL = = = BC 2 106 106 13 106 units ⇒ AL = 53

Example 1.17 The vertices of a triangle ABC are A(x1, x1 tana), B(x2, x2 tanb  ) and C(x3, x3 tang  ). If H ( x , y ) is the orthocentre and S(0,0) is the circumcentre, then show that y sin a + sin b + sin g = . x cos a + cos b + cos g A

S G B

A′

H D

C

Solution If r is the circumradius of D ABC, SA = SB = SC = r, SA2 = r2 (i.e.)

x12 + x12 tan2a = r2

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1-16

Analytical Geometry

(i.e.)

x12(1 + tan2a) = r2 ⇒x1 = rcosa

Similarly,

x2 = rcosb and x3 = rcosg

Then, the coordinates of A, B and C are (rcosa, rsina), (rcosb, rsinb ) and  cos a + cos b + cos g , (rcosg, rsing ). The centroid of the triangle is G   3 sin a + sin b + sin g   . 3 The orthocentre is H ( x , y ) and the circumcentre is S(0,0). Geometrically we know that H, G and S are collinear. Therefore, the slope of SG and GH are equal.

∴

sin a + sin b + sin g y = cos a + cos b + cos g x

Example 1.18 A line joining the two points A(2, 0) and B(3, 1) is rotated about A in the anticlockwise direction through an angle of 15°. If B goes to C in the new position, find the coordinate of C.

Solution y C (x1,y1) 15° O

A(2,0)

B (3,1) 45°

L

x

Given that C is the new position of B. Draw CL perpendicular to OX and let (x1, y1) be the coordinates of C. Now



AB = (1 − 0)2 + (3 − 2)2 = 2

Slope of

AB = 1

AB makes 45° with x-axis and CAC = 60°. Then

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Coordinate Geometry

1-17

OL = OA + AL = 2 + AC cos 60° 1 2 4+ 2 = 2+ = 2 2 2 2× 3 6 = LC = AC sin 60° = 2 2  4+ 2 6 Hence, the coordinates of C are  , . 2   2 = 2+ 2 ×



Example 1.19 The coordinates of A, B and C are (6, 3), (-3, 5) and (4, -2), respectively, and P is any point (x, y). Show that the ratio of the area of D PBC and D ABC is x+ y−2 . 7

Solution The points A, B, C and P are (6, 3), (-3, 5), (4, -2) and (x, y), respectively. 1 Area of ∆ ABC = [6(5 + 2) − 3( −2 − 3) + 4(3 − 5)] 2 1 49 = [42 + 15 − 8] = square units 2 2 1 Area of ∆ PBC = [ x(5 + 2) − 3( −2 − y ) + 4( y − 5)] 2 1 1  7 x + 7 y − 14  = [7 x + 6 + 3 y + 4 y − 20] =   2 2 1  x+ y−2 Area of ∆ PBC 1 7 x + 7 y − 14 = = 49 7 Area of ∆ ABC 2 2

Example 1.20 Find the coordinates of the point that divides the line joining the points (2, 3) and (-4, 7) (i) internally (ii) externally in the ratio 3:2.

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1-18

Analytical Geometry

Solution Let R and R′ respectively divide PQ internally and externally in the ratio 3:2. P (2, 3)

R

Q (−4, 7)

R′

 lx + mx1 ly2 + my1  , (i) The coordinates of R are  2  l+m l + m  (i.e.) (i.e.)

 3 × ( −4) + 2 × 2 3 × 7 + 2 × 3  ,   3+ 2  3+ 2  −8 27   ,  5 5

 3 × ( −4) − 2 × 2 3 × 7 − 2 × 3  (ii) The coordinates of R′ are  ,   3−2 3−2  (i.e.)   (-16, 15)

Example 1.21 Find the ratio in which the line joining the points (4, 7) and (-3, 2) is divided by the y‑axis.

Solution Let the y-axis meet the line joining the joints P(4, 7) and Q(-3, 2) at R. Let the coordinates of R be (0, y). Let R divide PQ in the ratio k:1. k ( −3) + 1 × 4 4 = 0.  ∴ 3k = 4 ⇒ k = The coordinates of R is given by k +1 3 Hence, the ratio in which R divides PQ is 4:3.

Example 1.22 Show that the points (-2, -1), (1, 0), (4, 3) and (1, 2) form the vertices of a parallelogram.

Solution A(−2, −1)

D(1, 2)

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B(1, 0)

C(4, 3)

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Coordinate Geometry

1-19

A quadrilateral in which the diagonals bisect each other is a parallelogram.  x + x y + y2   −2 + 4 −1 + 3  (i.e.)  , The midpoint of AC is  1 2 , 1  (i.e.) (1,1).  2  2 2  2   1+1 2 + 0 , The midpoint of BD is   (i.e.) (1,1).  2 2  Since the diagonals bisect each other, ABCD is a parallelogram.

Example 1.23 Find (x, y) if (3, 2), (6, 3), (x, y) and (6, 5) are the vertices of a parallelogram taken in order.

Solution Let the four points be A, B, C and D, respectively. Since ABCD is a parallelogram, the midpoint of AC is the same as the midpoint of BD.  3+ x 2 + y , The midpoint of AC is   . The midpoint of BD is  2 2   6 + 6 3 + 5 ,   (i.e.) (6, 4). 2 2  2+4 3+ x ∴ = 6 and = 4 ⇒ x = 9, y = 6. 2 2 Hence (x, y) is (9, 6).

Example 1.24 The midpoints of the sides of a triangle are (6, -1), (-1, -2) and (1, 4). Find the coordinates of the vertices.

Solution A(x1, y1)

F(1, 4)

B(x2, y2)

E(−1, −2)

D(6, −1)

C(x3, y3)

Let D, E and F be the midpoints of the sides BC, CA and AB, respectively. Then, (6, -1), (-1, -2), (1, 4) are the points D, E and F, respectively.

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1-20

Analytical Geometry

Let A(x1, y1), B(x2, y2) and C(x3, y3) be the vertices of the triangle. Then BDEF is a parallelogram. 6 + 1 −1 + 4  The midpoint of DF is  ,   2 2 

 7 3 (i.e.)  ,  .  2 2

 x − 1 y2 − 2  The midpoint of BE is  2 , .  2 2  x2 − 1 7 y −2 3 = and 2 = ∴ x2 = 8, y2 = 5 ∴ B (8,5). 2 2 2 2  8 + x1 5 + y1  = (1, 4) , Since F is the midpoint of AB,   2 2 

∴

8 + x1 5 + y1 = 1, = 4 ⇒ x1 = −6, y1 = 3 ∴ A( −6, 3) 2 2 3 + y3 −6 + x3 = −1, = −2 Since E is the midpoint of AC, 2 2 \ x3 = 4, y3 = -7. \C is the point (4, -7). ∴

Hence, the vertices of the triangle are (-6, 3), (8, 5) and (4, -7).

Example 1.25 Show that the axes of coordinates trisect the straight line joining the points (2, -2) and (-1, 4).

Solution Let the line joining the points (2, -2) and (-1, 4) meet x-axis and y-axis at A and B, respectively. Let the coordinates of A and B be (x, 0) and (0, y), respectively. Let A divide the line in the ratio k:1. Then the x-coordinate of A k ( −1) + 1 × 2 = 0. is given by k +1 y B A x′

O

x y′

\ -k + 2 = 0 ⇒ k = 2. Hence, A divides the line in the ratio 2:1.

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Coordinate Geometry

1-21

Let B divide the line in the ratio l:1. Then, λ( 4) + 1(2) = 0. 1 λ +1 Hence, 4l = 2 ⇒ λ = 2 B divides the line in the ratio 1:2. Hence, A and B trisect the line joining the points (2, -4) and (-1, 4).

Example 1.26 The vertices of a triangle are A(3, 5), B(-7, 9) and C(1, -3). Find the length of the three medians of the triangle.

Solution A(3, 5) E

F B(−7, 9)

D

C(1, −3)

Let D, E and F be the midpoints of the sides of BC, CA and AB, respectively.  −7 + 1 9 − 3  , The coordinates of D are   (i.e.) (-3, 3). The coordinates of E are  2 2   3 + 1 5 − 3  3 − 7 5 + 9 , ,   (i.e.) (2, 1). The coordinates of F are   (i.e.) (-2, 7). 2 2 2 2  AD 2 = (3 + 3)2 + (5 − 3)2 = 36 + 4 = 40 BE 2 = ( −7 − 2)2 + (9 − 1)2 = 81 + 64 = 145 CF 2 = (11 + 2)2 + ( −3 − 7)2 = 9 + 100 = 109 Hence,

the

lengths

of

the

medians

are

AD = 2 10, BE = 145

and CF = 109 units.

Example 1.27 Two of the vertices of a triangle are (4, 7) and (-1, 2) and the centroid is at the origin. Find the third vertex.

Solution Let the third vertex of the triangle be (x, y). Then



4 −1+ x 7+2+ y = 0 and = 0 ∴ x = −3, y = −9. 3 3

Hence, the third vertex is (-3, -9).

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1-22

Analytical Geometry

Example 1.28 Show that the midpoint of the hypotenuse of the right angled triangle whose vertices are (8, -10), (7, -3) and (0, -4) is equidistant from the vertices.

Solution Let the three given points be A(8, -10), B(7, -3) and C(0, -4).

AB2 = (8, 7)2 + (-10 + 3)2 = 1 + 49 = 50



BC2 = (7 - 0)2 + (-3 + 4)2 = 49 + 1 = 50



AC2 = (8 - 0)2 + (-10 + 4)2 = 64 + 36 = 100 A(8, −10)

B(7, −3)

C(0, −4)

AB2 + BC2 = AC2 Hence, ABC is a right angled triangle with AC as hypotenuse.  8 + 0 −10 − 4  , The midpoint of AC is P   (i.e.) (4, -7).  2 2 

PA2 = (8 - 4)2 + (-10 + 7)2 = 16 + 9 = 25



PB2 = (7 - 4)2 + (-3 + 7)2 = 9 + 16 = 25



PC2 = (0 - 4)2 + (-4 + 7)2 = 16 + 9 = 25



PA = PB = PC

Hence, the midpoint of the hypotenuse is equidistant from the vertices.

Example 1.29 Find the ratio in which the line joining the points (1, -1) and (4, 5) is divided by the point (2, 1).

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Coordinate Geometry

1-23

Solution (1, −1)

(2, 1)

(4, 5)

R

R

R

Let the point R(2, 1) divide the line joining the points P(1, -1) and Q(4, 5) in the ratio k:1. Then ( k × 4) + (1 × 1) = 2 ⇒ 4 k + 1 = 2k + 2 k +1 1 ⇒ 2k = 1 ⇒ k = 2 Therefore, R(2, 1) divides PQ in the ratio 1:2.

Example 1.30 Find the locus of the point that is equidistant from two given points (2, 3) and (-4, 1).

Solution Let P(x, y) be a point such that PA = PB where A and B are the points (2, 3) and (-4, 1), respectively. PA2 = PB2 ⇒ (x - 2)2 + (y - 3)2 = (x + 4)2 + (y - 1)2



⇒ x2 + 4 - 4x + y2 - 6y + 9 = x2 + 16 + 8x + y2 - 2y + 1

⇒ 12x + 4y = 4 or 3x + y = 1

Example 1.31 Find the locus of the point that moves from the point (4, 3) keeping a constant distance of 5 units from it.

Solution Let C(4, 3) be the given point and P(x, y) be any point such that CP = 5. Then

CP2 = 25 ⇒(x - 4)2 + (y - 3)2 = 25



⇒ x2 + y2 - 8x - 6y = 0. This is the required locus.

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1-24

Analytical Geometry

Example 1.32 The ends of a rod of length l move on two mutually perpendicular lines. Show that the locus of the point on the rod that divides it in the ratio 1:2 is 9x2 + 36y2 = l2.

Solution Let AB be a rod of length l whose ends A and B are on the coordinate axes. Let the coordinates of A and B be A(a, 0) and B(0, b). Let the point P(x1, y1) divide AB in the ratio 1:2. y B(0,b) 2

P(x1, y1) 1

O

A(a,0)

x

2a b  2a b  Then the coordinates of P are  ,  ∴ x1 = , y1 =  3 3 3 3 3 x1 and b = 3 y1 (i.e) a = 2 2  3x  OA2 + OB 2 = AB 2 ⇒ a 2 + b 2 = l 2 ⇒  1  + (3 y1 )2 = l 2  2  or

9x12 + 36 y12 = l 2

Hence, the locus of (x1, y1) is 9x2 + 36y2 = l2.

Example 1.33 A point moves such that the sum of its distances from two fixed points (al, 0) and (-al, 0) is always 2a. Prove that the equation of the locus is x2 y2 + 2 = 1. 2 a a (1 − l 2 )

Solution Let the two fixed points be A(al, 0) and B(0, -al). Let P(x1, y1) be a moving point such that PA + PB = 2a. Given that

( x1 − al )2 + y12 + ( x1 + al )2 + y12 = 2a.

M01_ANALYTICALGEOMETRY_ISBN_CH01.indd 24

(1.10)

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Coordinate Geometry

1-25

Then ( x1 − al )2 + y12  − ( x1 + al )2 + y12  = 2a  ( x1 − al )2 + y12 − ( x1 + al )2 + y12       

−4 ax1l = 2a  ( x1 − al )2 + y12 − ( x1 + al )2 + y12    or

( x1 − al )2 + y12 − ( x1 + al )2 + y12 = −2 x1l

(1.11)

Adding (1.10) and (1.11), 2 ( x1 − al )2 + y12 = 2( a − lx1 ) Squaring on both sides, we get

( x1 − al )2 + y12 = ( a − lx1 )2 (i.e.) x12 (1 − l 2 ) + y12 = a 2 (1 − l 2 )

Dividing by a2(1 - l2), we get x12 y2 + 2 1 2 =1 2 a a (1 − l ) Therefore, the locus of (x1, y1) is

x2 y2 + 2 = 1. 2 a a (1 − l 2 )

Example 1.34 A right angled triangle having the right angle at C with CA = a and CB = b moves such that the angular points A and B slide along the x-axis and y-axis, respectively. Find the locus of C.

Solution Let the points A and B be on the x-axis and y-axis, respectively. Let A and B have coordinates (a, 0) and (0, b ). Let C be the point with coordinates (x1, y1). y B(0, b )

b

a

b O

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C

L α

A(a, 0) x

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1-26

Analytical Geometry

Then

( x1 − a ) 2 + y12 = a 2

(1.12)

x12 + ( y1 − b )2 = b 2

(1.13)

Then AB2 = a2 + b2. Also AB2 = a 2 + b 2. \ a2 + b2 = a 2 + b 2

(1.14)

a = x1 − a 2 − y12



b = y1 − b 2 − x12 Hence, a 2 + b  2 = a2 + b2

(

⇒ x1 − a 2 − y12

) +(y − 2

1

b 2 − x12

)

2

= a2 + b2

⇒ 2 x1 a 2 − y12 − 2 y1 b 2 − x12 = 0 or

x12 ( a 2 − y12 ) = y12 (b 2 − x12 ) ⇒ a 2 x12 − b 2 y12 = 0

Hence, the locus of c(x1, y1) is a2x2 - b2y2 = 0.

Example 1.35 Two points P and Q are given. R is a variable point on one side of the line PQ such that RPQ − RQP is a positive constant 2a. Find the locus of the point P.

Solution Let PQ be the x-axis and the perpendicular through the midpoint of PQ be the y-axis. Let P and Q be the points (a, 0) and (-a, 0), respectively. Let R be the point (x1, y1). Let RPQ = q and RQP = f . Then RPQ − RQP = 2a . y R(x1, y1) y1 q

∅

Q

M01_ANALYTICALGEOMETRY_ISBN_CH01.indd 26

O

x1

L

P (a, 0)

x

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1-27

Coordinate Geometry

(i.e.)  q - f = 2a. Then tan(q - f) = tan2a tan q − tan f = tan 2a 1 + tan q tan f



y1 y − 1 a − x1 a + x1 ⇒ = tan 2α y y 1+ 1 . 1 a − x1 a + x1 ⇒



y1 ( a/ + x1 − a/ + x1 ) = tan 2a a 2 − x12 + y12

⇒ 2 x1 y1 = ( a 2 − x12 + y12 ) tan 2a or x12 − y12 + 2 x1 y1 cot 2a = a 2

Hence, the locus of (x1, y1) is x2 - y2 - 2xy cot 2a = a2.

Exercises 1.

2.

Show that the area of the triangle with vertices (a, b), (x1, y1) and (x2, y2) where a, x1 and x2 are in geometric progression with common ratio r and b, y1 and y2 1 are in geometric progression with common ratio s is ab ( r − 1)( s − 1)(b − r ) . 2 If P(1, 0), Q(-1, 0) and R(2, 0) are three given points, then show that the locus of the point S satisfying the relation SQ2 + SR2 - 2SP2 is a straight line parallel to the y-axis.

3.

Show that the points (p + 1, 1), (2p + 1, 3) and (2p + 2, 2) are collinear if p = 2 or −1 . 2

4.

Show that the midpoint of the vertices of a quadrilateral coincides with the midpoint of the line joining the midpoint of the diagonals.

5.

Show that if t1 and t2 are distinct and nonzero, then (t12 , t1 ),(t22 , t2 ) and (0, 0) are collinear.

6.

If the points

 a3 a2 − 3   b3 b3 − 3   c3 c3 − 3 are collinear  a − 1 , a − 1  ,  b − 1 , b − 1  and  c − 1 , c − 1  for three distinct values a, b and c, then show that abc - (bc + ca + ab) + 3(a + b + c)  = 0.

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1-28

Analytical Geometry

7.

Perpendicular straight lines are drawn through the fixed point C(a, a) to meet the axes of x and y at A and B. An equilateral triangle is described with AB as the base of the triangle. Prove that the equation of the locus of C is the curve y2 = 3(x2 + a2).

8.

The ends A and B of a straight line segment of constant length c slides upon the fixed rectangular axes OX and OY, respectively. If the rectangle OAPB is completed, then show that the locus of the foot of the perpendicular drawn 2 2 2 from P to AB is x 3 + y 3 = c 3 .

9.

The point A divides the line joining P(1, -5) and Q(3, 5) in the ratio k:1. Find the two values of k for which the area of the triangle ABC is equal to 2 units in magnitude when the coordinates of B and C are (1, 5) and (7, -2), respectively.

10.

The line segment joining A(3, 0) and B(0, 2) is rotated about a point A in the anticlockwise direction through an angle of 45° and thus B moves to C. If point D be the reflection of C in the y-axis, find the coordinates of D.

(

Ans.: −3, 2 2



)

11.

If (a, b), (h, k) and (p, q) be the coordinates of the circumcentre, the centroid and the orthocentre of a triangle, prove that 3h = p + 2a.

12.

Prove that in a right angled triangle, the midpoint of the hypotenuse is equidistant from its vertices.

13.

If G is the centroid of a triangle ABC, then prove that 3(GA2 + GB2 + GC2) = AB2 + BC2 + CA2.

14.

Show that the line joining the midpoint of any two sides of a triangle is half of the third side.

15.

Prove that the line joining the midpoints of the opposite sides of a quadrilateral and the line joining the midpoints of the diagonals are concurrent.

16.

If D1 and D2 denote the area of the triangles whose vertices are (a, b), (b, c), (c, a) and (bc - a2, ca - b2), (ca - b2, ab - c2) and (ab - c2, bc - a2), respectively, then show that D2 = (a + b + c)2D1.

17.

Prove that if two medians of a triangle are equal, the triangle is isosceles.

18.

If a, b and c be the pth, qth and rth terms of a HP, then prove that the points having coordinates (ab, r), (bc, p) and (ca, q) are collinear.

19.

Prove that a point can be found that is at the same distance from each of the  a   a  a  a four points  at1 ,   at2 ,  ,  at3 ,  and  ,at1t2t3  . t  t   t   tt t 

20.

If (x1, y1) (x2, y2) (x3, y3) and (x4, y4) be the vertices of a parallelogram and x1 x3 + y1y3 = x2x1 + y2y1 then prove that the parallelogram is a rectangle.

1

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2

3

1 2 3

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1-29

Coordinate Geometry

21.

In any DABC, prove that AB2 + AC2 = 2(AD2 + DC2) where D is the midpoint of BC.

22.

If G is the centroid of a triangle ABC and O be any other point, then prove that

23.

(i)

AB2 + BC2 + CA2 = 3(GA2 + GB2 + GC2 )

(ii)

OA2 + OB2 + OC2 = GA2 + GB2 + GC2 + 3GO2

Find the incentre of the triangle whose vertices are (20, 7), (-36, 7) and (0, -8).

(

Ans.: 2 2 , 2 − 2 2

 24.

If A, B and C are the points (-1, 5), (3, 1) and (5, 7), respectively, and D, E and F are the midpoints of BC, CA and AB, respectively, prove that area of D ABC is four times that of D DEF.

25.

If D, E and F divide the sides BC, CA and AB of D ABC in the same ratio, prove that the centroid of D ABC and D DEF coincide.

26.

A and B are the fixed points (a, 0) and (-a, 0). Find the locus of the point P that moves in a plane such that (i)

PA2 + PB2 = 2k2

(ii)

PA2 - PB2 = 2PC2 where C is the point (c, 0)

)

Ans.: (i) 2ax + k2 = 0 (ii) 2cx = c2 - a2

  27.

If (xi, yi), i = 1, 2, 3 are the vertices of the D ABC and a, b and c are the lengths of the sides BC, CA and AB, respectively, show that the incentre of the triangle ax + bx2 + cx3 ay1 + by2 + cy3  ABC is  1 , .  a+b+c a+b+c 

28.

Show that the points (-a, -b), (0, 0), (a, b) and (a2, b2) are either collinear, the vertices of a parallelogram or the vertices of a rectangle.

29.

The coordinates of three points O, A and B are (0, 0), (0, 4) and (6, 0), respectively. A point P moves so that the area of D POA is always twice the area of D POB. Find the equation of the locus of P. Ans.: x2 - 9y2 = 0

 30.

31.

The four points A(x1, 0), B(x2, 0), C(x3, 0) and D(x4, 0) are such that x1, x2 are the roots of the equation ax2 + 2hx + b = 0 and x3, x4 are the roots of the equation a1x2 + 2h1x + b1 = 0. Show that the sum of the ratios in which C and D divide AB is zero, provided ab1 + a1b = 2hh1. x1 If x2 x3

y1 1 a1 y2 1 = a2 y3 1 a3

b1 1 b2 1 , then show that the triangle with vertices (xi, yi), b3 1

i = 1, 2, 3 and (ai, bi), i = 1, 2, 3 are congruent.

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1-30

32.

Analytical Geometry

The point (4, 1) undergoes the following three transformations successively: (i) Reflection about the line y = x (ii) Transformation through a distance of 2 units along the positive direction of x-axis p (iii) Rotation through an angle of about the origin in the anticlockwise 4 direction.



Find the final position of the point.

33.

Show that the points P(2, -4) , Q (4, -2) and R (1, 1) lie on a straight line. Find (i) the ratio PQ:QR and (ii) the coordinates of the harmonic conjugation of Q with respect to P and R.

34.

If a point moves such that the area of the triangle formed by that point and the points (2, 3) and (-3, 4) is 8.5 square units, show that the locus of the point is x + 5y - 34 = 0.

35.

Show that the area of the triangle with vertices (p + 5, p - 4), (p - 2, p + 3) and (p, p) is independent of p.

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Chapter

2

The Straight Line

2.1  INTRODUCTION In the previous chapter, we defined that the locus of a point is the path traced out by a moving point according to some geometrical law. We know that the locus of a point which moves in such a way that its distance from a fixed point is always constant.

2.1.1  Determination of the General Equation of a Straight Line Suppose the point P(x, y) moves such that P(x, y), A(4, -1), and B(2, 3) form x y 1 a straight line. Then, ∆ = 4 −1 1 = 0 2 3 1

⇒ x( −4) − y(2) + 14 = 0

(i.e.) 4x + 2y – 14 = 0 or 2x + y - 7 = 0, which is a first degree equation in x and y that represents a straight line. The general equation of a straight line is ax + by + c = 0. Suppose ax + by + c = 0 is the locus of a point P(x, y). If this locus is a straight line and if P(x1, y1) and Q(x2, y2) be any two points on the locus then the point R which divides PQ with ratio l :1 is also a point on the line. Since P(x1, y1) and Q(x2, y2) lie on the locus ax + by + c = 0,

ax1 + by1 + c = 0

(2.1)



ax2 + by2 + c = 0

(2.2)

On multiplying equation (2.2) by l and adding with equation (2.1), we get

l (ax2 + by2 + c) + (ax1 + by1 + c) = 0.

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2-2

Analytical Geometry



(i.e.)  a(l x2 + x1) + b(k y2 + y1) + c(l + 1) = 0.

On dividing by l + 1, we get

 l y + y1   lx + x  + c = 0. a 2 1 + b 2  l + 1   l +1 

(2.3)

 lx + x ly + y1  Equation (2.3) shows that the point  2 1 , 2 lies on the locus  l +1 l + 1  ax + by + c = 0. This shows that the point which divides PQ in the ratio l:1 also lies on the locus which is the definition for a straight line. \ ax + by + c = 0 always represents a straight line. a b Note 2.1.1.1: The above equation can be written in the form x + y + 1 = 0 c c which is of the form Ax + By + 1 = 0. Hence, there are two independent constants in equation of a straight line. Now, we look into various special forms of the equation of a straight line.

2.1.2  Equation of a Straight Line Parallel to y-axis and at a Distance of h units from x-axis Let PQ be the straight line parallel to y-axis and at a constant distance h units from y-axis. Then every point on the line PQ has the x-coordinate h. Hence the equation of the line PQ is x = h. y P

O

x

h Q

Note 2.1.2.1: 1. Similarly, the equation of the line parallel to x-axis and at a distance k from it is y = k. 2. The equation of x-axis is y = 0. 3. The equation of y-axis is x = 0.

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The Straight Line

2-3

2.2  SLOPE OF A STRAIGHT LINE If a straight line makes an angle q  with the positive direction of x-axis then tan q  is called the slope of the straight line and is denoted by m.

∴m = tan q. y

q

x

O

We can now determine the slope of a straight line in terms of coordinates of two points on the line. Let P(x1, y1) and Q(x2, y2) be the two given points on a line. Draw PL and QM perpendiculars to x-axis. Let PQ make an angle q  with OX. y P(x1, y1)

(x2, y2) Q q q O

M

R L

x

Draw QR perpendicular to LP. Then RQP = q .

\  QR = ML = OL - OM = x1 - x2.

In ∆PQR, tanq =

RP = LR - LP = LP - QM = y1 - y2. y − y2 RP   (i.e.)  m = 1 x1 − x2 QR

2.3  SLOPE-INTERCEPT FORM OF A STRAIGHT LINE Find the equation of the straight line, which makes an angle p  with OX and cuts off an intercept c on the y-axis.

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2-4

Analytical Geometry

y

P

x′

B q q c A O

N x

L

y′

Let P(x, y) be any point on the straight line which makes an angle q with x-axis. OAP = q , OB = c = y-intercept. Draw PL perpendicular to x-axis and BN perpendicular to LP. Then, NBP = q . BN = OL = x.

∴  NP = LP - LN = LP - OB = y - c.

NP y−c   (i.e.)  m = ⇒ y = mx + c. BN x This equation is true for all positions of P on the straight line. Hence, this is the equation of the required line.

In ∆NBP , tanq =

2.4  INTERCEPT FORM Find the equation of the straight line, which cuts off intercepts a and b, respectively on x and y axes. y

B b O

P (x, y) N a A

x

Let P(x, y) be any point on the straight line which meets x and y axes at A and B, respectively. Let OA = a, OB = b, ON = x, and NP = y; NA = OA - ON = a - x. Triangles PNA and BOA are similar. Therefore, PN NA y a−x y x x y = (i.e.) = ⇒ = 1 − or + = 1. This result is true for OB OA b a b a a b

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The Straight Line

2-5

all positions of P on the straight line and hence this is the equation of the required line.

2.5  SLOPE-POINT FORM Find the equation of the straight line with slope m and passing through the given point (x1, y1 ). The equation of the straight line with a given slope m is y = mx + c (2.4) Here, c is unknown. This straight line passes through the point (x1, y1). The point has to satisfy the equation y = mx + c. ∴ y1 = mx1 + c. Substituting the value of c in equation (2.4), we get the equation of the line as

y = mx + y1 − mx1 ⇒ y − y1 = m( x − x1 ).

2.6  TWO POINTS FORM Find the equation of the straight line passing through two given points (x1, y1) and (x2, y2). y - y1 = m(x - x1)



(2.5)

where, m is unknown. The slope of the straight line passing through the points

(x1, y1) and (x2, y2) is m =

y1 − y2 x1 − x2

(2.6)

By substituting equation (2.6) in equation (2.5), we get the required straight y − y2 line y − y1 = 1 ( x − x1 ). x1 − x2

(i.e.) 

y − y1 y1 − y2 = . x − x1 x1 − x2

2.7  NORMAL FORM Find the equation of a straight line in terms of the perpendicular p from the origin to the line and the angle that the perpendicular line makes with axis.

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2-6

Analytical Geometry

y B a L p a

x

A

O

Draw OL ⊥ AB. Let OL = p. Let AOL = a

∴ OBA = a OA = seca OL OA = OL sec a = p sec a OB = coseca OL ∴ OB = OL cosec a = p cosec a .

Therefore, the equation of the straight line AB is

x y + = 1. p sec a p cosec a

(i.e.)  x cos a + y sin a = p

2.8  PARAMETRIC FORM AND DISTANCE FORM Let a straight line make an angle q with x-axis and A(x1, y1) be a point on the line. Draw AL, PM perpendicular to x-axis and AQ perpendicular to PM. Then, PAQ = q . y ) r , y1 1 q x (

A

q O

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L

P

(x,

y)

Q

M

x

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2-7

The Straight Line



AQ = LM = OM - OL = x - x1.



QP = MP - MQ = MP - LA = y - yl. Let AP = r.

In ∆PAQ, x - x1 = rcosq ; y - y1 = rsinq. x − x1 y − y1 = = r. cos q sin q These are the parametric equations of the given line. ∴



Note 2.8.1: Any point on the line is x = x1 + rcosq, y = y1 + rsinq. Note 2.8.2: r is the distance of any point on the line from the given point A(x1, y1).

2.9  PERPENDICULAR DISTANCE ON A STRAIGHT LINE Find the perpendicular distance from a given point to the line ax + by + c = 0. y B Q R

T S p p1 a

O

P

A

x

Let R(x1, y1) be a given point and ax + by + c = 0 be the given line. Through R draw the line PQ parallel to AB. Draw OS perpendicular to AB meeting PQ at T. Let OS = p and PT = p1. Let AOS = a . Then the equation of AB is

x cos a + y sin a = p

(2.7)

ax + by = −c

(2.8)

which is the same as

Equations (2.7) and (2.8) represent the same line and, therefore, identifying we get



cos a sin a − p cos 2 a + sin 2 a = = = ± a b c a2 + b2 ∴ cos a = ±

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a a +b 2

2

; sin a = ±

b a + b2 2

.

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2-8

Analytical Geometry

c

p=∓



. a + b2 The equation of the line PQ is x cos a + y sin a = p. Since the point R(x1, y1) lies on the line x 1 cos a + y1 sin a − p1 = 0. 2

∴ p1 = x1 cos a + y1 sin a .



Then, the length of the perpendicular line from R to AB = p − p1 = p − x1 cos a − y1 sin a = ∓ =∓

ax1 + by1 + c a2 + b2

=

c a +b 2

2

∓

ax1 a +b 2

2

∓

by1 a2 + b2

ax1 + by1 + c a2 + b2

Note 2.9.1: The perpendicular distance from the origin on the line ax + by + c = 0 is

c a + b2 2

.

2.10  INTERSECTION OF TWO STRAIGHT LINES Let the two intersecting straight lines be a1x + b1 y + c1 = 0 and a2x + b2 y + c2 = 0. Let the straight lines intersect at the point (x1, y1). Then (x1, y1) lies on both the lines and hence satisfy these equations. Then a1x1 + b1 y1 + c1 = 0  and  a2 x2 + b2 y2 + c2 = 0. Solving the equations, we get x1 y 1 = = b1c2 − b2 c1 c1a2 − c2 a1 a1b2 − a2 b1  bc −b c c a −c a  Therefore, the point of intersection is  1 2 2 1 , 1 2 2 1  .  a1b2 − a2 b1 a1b2 − a2 b1  Find the ratio at which the line ax + by + c = 0 divides the line joining the points (x1, y1) and (x2, y2 ). P (x1, y1) l R

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ax + by + c = 0

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The Straight Line

2-9

Let the line ax + by = c = 0 divide the line joining the points P(x1, y1) and Q(x2, y2) in the ratio l:1. Then, the coordinates of the point of division R are  x1 + l x2 y1 + l y2   1 + l , 1 + l  . This point lies on the line ax + by + c = 0.

 y + ly2   x + l x2  + c = 0. + b 1 ∴a 1  1 + l   1 + l 



(i.e.)  a (x1 + lx2) + b (y1 + ly2) + c (1 + l ) = 0.



ax1 + by1 + c + l (ax2 + by2 + c2) = 0.



⇒l =−

ax1 + by1 + c . ax2 + by2 + c2

Note 2.10.1: 1. If l is positive then the points (x1, y1) and (x2, y2) lie on the opposite sides of the line ax + by + c = 0. 2. If l is negative then the points (x1, y1) and (x2, y2) lie on the same side of the line ax + by + c = 0. 3. In other words, if the expressions ax1 + by1 + c and ax2 + by2 + c2 are of opposite signs then the point (x1, y1) and (x2, y2) lie on the opposite sides of the line ax + by + c = 0. If they are of the same sign then the points (x1, y1) and (x2, y2) lie on the same side of the line ax + by + c = 0. Find the equation of a straight line passing through intersection of the lines a1x + b1 y + c = 0 and a2x + b2 y + c = 0. Consider the equation a1x + b1y + c1 + l (a2x + b2 y + c2) = 0. (2.9) This is a linear equation in x and y and hence this equation represents a straight line. Let (x1, y1) be the point of intersection of the lines a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0. Then (x1, y1) has to satisfy the two equations:

a1x1 + b1 y1 + c1 = 0

(2.10)



a2x2 + b2 y2 + c2 = 0

(2.11)

On multiplying equation (2.11) by l and adding with equation (2.10) we get,

(a1x1 + b1 y1 + c1) + l (a2x2 + b2 y2 + c2) = 0.

This equation shows that the point x = x1 and y = y1 satisfies equation (2.9). Hence the point (x1, y1) lies on the straight line given by the equation (2.9),

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2-10

Analytical Geometry

which is a line passing through the intersection of the lines a1x + b1y + c1 = 0 and a2 x + b2y + c2 = 0.

2.11  CONCURRENT STRAIGHT LINES Consider three straight lines given by equations:

a1 x + b1 y + c1 = 0

(2.12)



a2 x + b2 y + c2 = 0 a3x + b3 y + c3 = 0

(2.13) (2.14)

The point of intersection of lines given by equations (2.12) and (2.13) is  b1c2 − b2 c1 c1c2 − c2 a1   a b − a b , a b − a b  . 1 2 2 1 1 2 2 1 If the three given lines are concurrent, the above point should lie on the straight line given by equation (2.14).

b c −b c  c a −c a  Then, a3  1 2 2 1  +  1 2 2 1  b3 + c3 = 0.  a1b2 − a2 b1   a1b2 − a2 b1 



(i.e.)  a3(b1c2 - b2c1) + b3(c1a2 - c2a1) + c3(a1b2 - a2b1) = 0.

This is the required condition for the three given lines to be concurrent. The a1 b1 c1 above condition can be expressed in determinant form a2 b2 c2 = 0. a3 b3 c3 If l, m, and n are constants such that l(al x + b1 y + c1 ) + m(a2 x + b2 y + c2) + n(a3 x + b3 y + c3 ) vanishes identically then prove that the lines a1 x + b1 y + c1 = 0, a2 x + b2 y + c2 = 0, and a3 x + b3 y + c3 = 0 are concurrent. Let the lines a1x + b1 y + c1 = 0 and a2 x + b2 y + c2 = 0 meet at the point (x1, y1).

Then  a1x1 + b1 y1 + c1 = 0

(2.15)



and  a2x2 + b2 y2 + c2 = 0

(2.16)

For all values of x and y given that,

l(a1x + b1 y + c1) + m(a2 x + b2 y + c2) + n(a3 x + b3 y + c3) = 0 (2.17)

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The Straight Line

2-11

Then it will be true for x = x1 and y = y1. \ l(a1x1 + b1y1 + c1) + m (a2x1 + b2 y1 + c2) + n (a3x1 + b3 y1 + c3) = 0. Using equations (2.15) and (2.16), we get a3x1 + b3 y1 + c3 = 0. That is, the point (x1, y1) lies on the line a3x + b3 y + c3 = 0. Therefore, the lines a1x + b1 y + c1 = 0, a2x + b2 y + c2 = 0, a3x + b3 y + c3 = 0 are concurrent at (x1, y1).

2.12  ANGLE BETWEEN TWO STRAIGHT LINES y

q q2 O

q1

x

Let q  be the angle between two straight lines, whose slopes are m1 and m2. Let the two lines with slopes m1 and m2 make angles q1 and q2 with x-axis. Then, m1 = tan q1, m2 = tan q2. Also, q = q1 - q2

tan q = tan(q 1 − q 2 ) =

( m − m2 ) tan q1 − tan q 2 = 1 . 1 + tan q1 tan q 2 1 + m1m2

If the RHS is positive, then q is the acute angle between the lines. If RHS is negative, then q is the obtuse angle between the lines.

∴ tan q = ±

 ( m − m2 )  ( m1 − m2 ) ⇒ q = tan −1  ± 1 1 + m1m2  1 + m1m2 

Note 2.12.1: If the lines are parallel then q  = 0 and tan q  = tan 0 = 0. ∴



m1 − m2 = 0 ⇒ m1 = m2 . 1 + m1m2

Note 2.12.2: If the lines are perpendicular then, q =

tan

p . 2

m − m2 p =± 1 ⇒ 1 + m1m2 = 0 ⇒ m1m2 = −1. 2 1 + m1m2

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2-12

Analytical Geometry

Therefore, (1)  If two lines are parallel then their slopes are equal. (2)  If the two lines are perpendicular then the product of their slopes is −1.

2.13  EQUATIONS OF BISECTORS OF THE ANGLE BETWEEN TWO LINES y

L A

D

, y)

R (x L′ P M B B1 C R′ x

O

Let AB and CD be the two intersecting straight lines intersecting at P. Let these lines be represented by the equations a1x + b1 y + c1 = 0 and a2x + b2 y + c2 = 0. Let PR and PR′ be the bisectors of angles APC and CPB, respectively. Then the perpendicular distances from R (or R′) AB, and CD are equal.

∴

a1 x + b1 y + c a +b 2 1

2 1

=±

a2 x + b2 y + c2 a 22 + b 22

If c1 and c2 are positive, then the equations of the bisector containing the origin is given by a1 x + b1 y + c1 a2 x + b2 y + c2 = (2.18) a 21 + b 21 a 22 + b 22 The equation of the bisector not containing the origin is a1 x + b1 y + c1 a x + b2 y + c2 =− 2 . (2.19) 2 2 a 1 +b 1 a 22 + b 22 If c1 and c2 are not positive then the equations of lines should be written in such a way that c1 and c2 are positive.

Note 2.13.1: We can easily observe that the two bisectors are at right angles.

ILLUSTRATIVE EXAMPLES Example 2.1 Find the equation of the straight line which is at a distance of 10 units from x-axis.

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The Straight Line

2-13

Solution y x = 10

O

10

x

The equation of the required straight line is x = 10 or x - 10 = 0.

Example 2.2 Find the equation of the straight line which is at a distance of −15 units from y-axis.

Solution y

O

x −15

The equation of the required line is y = −15 or y + 15 = 0.

Example 2.3 Find the slope of the line joining the points (2, 3) and (4, −5).

Solution The slope of the line joining the two given points (x1, y1) and (x2, y2) is y − y1 m= 2 . x2 − x1 Therefore, the slope of the line joining the two given points is −5 − 3 −8 m= = = −4. 4−2 2

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2-14

Analytical Geometry

Example 2.4 Find the slope of the line 2x - 3y + 7 = 0.

Solution The equation of the line is 2x − 3y + 7 = 0  (i.e.)  3y = 2x + 7. (i.e.)  y =



2 7 x+ . 3 3

2 Therefore, slope of the line = . 3

Example 2.5 Find the equation of the straight line making an angle 135° with the positive direction of x-axis and cutting of an intercept 5 on the y-axis.

Solution The slope of the straight line is m = tan q = tan 135° = tan(180 − 45) = −tan 45° = −1. y intercept = c = 5. Therefore, the equation of the straight line is

y = mx + c  (i.e.)  y = + 5 - x or x + y = 5.

Example 2.6 Find the equation of the straight line cutting off the intercepts 2 and −5 on the axes.

Solution y The equation of the straight line is  x + = 1. Here, a = 2 and b = -5. a b x y ­Therefore, the equation of the straight line is − = 1 or 5x − 2y = 10. 2 5

Example 2.7 Find the equation of the straight line passing through the points (7, −3) and cutting off equal intercepts on the axes.

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The Straight Line

2-15

Solution

x y + = 1. a b (i.e.)  x + y = a. This straight line passes through the point (7, −3). Therefore, 7 - 3 = a (i.e.) a = 4. \ The equation of the straight line is x + y = 4.

Let the equation of the straight line be

Example 2.8 Find the equation of the straight line, the portion of which between the axes is bisected at the point (2, -5).

Solution Let the equation of the straight line be

x y + = 1. a b

y

A(a, 0)

x′

x

(2

O ,−

5)

B(0, b)

y′

Let the line meet the x and y axes at A and B, respectively. Then the ­coordinates  a b of A and B are (a, 0) and (0, b). The midpoint of AB is  ,  . However, the  2 2 midpoint is given as (2, -5). a b Therefore, = 2 and = −5. 2 2

\ a = 4  and  b = −10. x y − = 1. 4 10 (i.e.)  5x - 2y = 20.

Hence, the equation of the straight line is

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2-16

Analytical Geometry

Example 2.9 Find the equation of the straight line of the portion of which between the axes is divided by the point (4, 3) in the ratio 2:3.

Solution Let the equation of the straight line be

x y + = 1. a b

y B(0, b) 3

(4, 3) 2

O

A(a, 0)

x

Let this line meet the x and y axes at A and B, respectively. The coordinates of A and B are (a, 0) and (0, b), respectively. The coordinates of the point that divides AB in the ratio 2:3 are

 2 × 0 + 3 × a 2 × b + 3 × 0 ,   5 5

 3a 2b  (i.e.)  ,   5 5

This point is given as (4, 3). 3a 2b = 4 and = 3. Therefore, 5 5

∴ a=

20 15 and b = . 3 2

\ The equation of the straight line is

3x 2 y + = 1. (i.e.) 9x + 8y = 60. 20 15

Example 2.10 Find the equations to the straight lines each of which passes through the point (3, 2) and intersect the x and y axes at A and B such that OA - OB = 2.

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The Straight Line

2-17

Solution y B(0, b) b O

a

A(a, 0)

x

x y Let the equation of the straight line be + = 1. This straight line passes a b through the point (3, 2).

3 2 ∴ + =1 a b



⇒ 3b + 2a = ab

(2.20)

Also, given that OA − OB = 2 (i.e.)  a - b = 2.



(2.21)

Therefore, b = a − 2. Substituting this in equation (2.20) we get 3(a − 2) + 2a = a(a - 2). 3a − 6 + 2a = a 2 − 2a

⇒ a 2 − 7a + 6 = 0 ⇒ ( a − 1)( a − 6) = 0.



\ a = 1 and a = 6. b = −1 and b = 4.

\ The two straight lines are

x y x y + = 1 and + = 1. 1 −1 6 4

(i.e.)  x - y = 1  and  2x + 3y = 12.

Example 2.11 Show that the points A(1, 1), B(5, -9), and C(−1, 6) are collinear.

Solution The slope of AB is m1 =

y1 − y2 1 + 9 10 5 = =− . = 2 x1 − x2 1 − 5 −4

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2-18

Analytical Geometry



∴ The slope of BC is m2 =

−9 − 6 −15 5 = =− . 5 +1 6 2

Since the slopes of AB and BC are equal and B is the common point, the points are collinear.

Example 2.12 Prove that the triangle whose vertices are (−2, 5), (3, -4), and (7, 10) is a right angled isosceles triangle. Find the equation of the hypotenuse.

Solution Let the points be A (−2, 5), B (3, −4), and C (7, 10). AB2 = (−2 − 3)2 + (5 + 4)2 = 25 + 81 = 106, BC2 = (3 − 7)2 + (−4 − 10)2 = 16 + 196 = 212. AC2 = (−2 − 7)2 + (5 − 10)2 = 81 + 25 = 106. Therefore, AB2 + AC2 = BC2 and AB = AC. Hence, the DABC is a right angled isosceles triangle. The equation of the hypotenuse BC is

y − y1 y1 − y2 = x − x1 x1 − x2

(i.e.)

y + 4 −4 − 10 −14 7 = = = x −3 −4 2 3−7

⇒ 7 x − 21 = 2 y + 8 ⇒ 7 x − 2 y − 29 = 0.

Example 2.13 Find the equation of the straight line which cuts off intercepts on the axes equal in magnitude but opposite in sign and passing through the point (4, 7).

Solution Let the equation of the straight line cutting off intercepts equal in magnitude x y but opposite in sign be − = 1 (i.e.) x − y = a. a b This passes through the point (4, 7). Therefore, 4 − 7 = a (i.e.) a = −3. Hence, the equation of the straight line is x − y + 3 = 0.

Example 2.14 Find the ratio in which the line 3x - 2y + 5 = 0 divides the line joining the points (6, -7) and (–2, 3).

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The Straight Line

2-19

1

k A (6, −7)

B (−2, 3)

Solution Let the line 3x − 2y + 5 = 0 divide the line joining the points A(6, -7) and B(−2, 3) in the ratio k:1. Then the coordinates of the point of division are  −2k + 6 3k − 7  ,   . Since this point lies on the straight line 3x − 2y + 5 = 0, k +1 k +1  we get  −2k + 6   3k − 7  3 − 2 + 5 = 0.  k + 1   k + 1  (i.e.)  -6k + 18 - 6k + 14 + 5k + 5 = 0. 37 (i.e.)  −7k + 37 = 0 ⇒ k = . 7 \ The required ratio is 37:7.

Example 2.15 Prove that the lines 3x – 4y + 5 = 0, 7x - 8y + 5 = 0, and 4x + 5y = 45 are concurrent.

Solution Given

3x - 4y = -5 7x - 8y = -5 4x + 5y = 45

(2.22) (2.23) (2.24)

Solving equations (2.22) and (2.23), we get the point of intersection of the two lines. (2.22) × 2 6 x − 8 y = −10 (2.23) × 1 7 x − 8 y = −5 x=5 \ From equation (2.22), 15 - 4y = - 5. \ y = 5. Hence, the point of intersection of the lines is (5, 5). Substituting x = 5 and y = 5, in equation (2.24), we get 20 + 25 = 45 which is true. \ The third line also passes through the points (5, 5). Hence it is proved that the three lines are concurrent.

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2-20

Analytical Geometry

Example 2.16 Find the value of a so that the lines x - 6y + a = 0, 2x + 3y + 4 = 0, and x + 4y + 1 = 0 are concurrent.

Solution x - 6y + a = 0 2x + 3y + 4 = 0 x + 4y + 1 = 0

Given

(2.25) (2.26) (2.27)

Solving the equations (2.26) and (2.27) we get,

(2.26) × 1 : 2 x + 3 y = −4 (2.27) × 2 : 2 x + 8 y = −2

On subtracting, we get 5 y = 2 ∴y=

\ From equation (2.27) x =

2 5

−8 −8 − 5 −1 = 5 5



\x =

−13 5

 −13 2  Hence, the point of intersection of the lines is  , . Since the lines are  5 5  concurrent this point should lie on x - 6y + a = 0.

(i.e.)

−13 12 − + a = 0. 5 5 ∴ a = 5.

Example 2.17 Prove that for all values of l the straight line x(2 + 3l) + y(3 - l) - 5 - 2l = 0 passes through a fixed point. Find the coordinates of the fixed point.

Solution x(2 + 3l) + y(3 - l) - 5 - 2l = 0. This equation can be written in the form

2x + 3y - 5 + l(3x - y - 2) = 0

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(2.28)

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The Straight Line

2-21

This equation represents a straight line passing through the intersection of lines

2x + 3y - 5 = 0 3x - y - 2 = 0

(2.29) (2.30)

for all values of l. (2.29)

2x + 3y = 5

(2.28) × 3

9x - 3y = 6

On adding, we get 11x = 11 ⇒ x = 1 and hence from equation (2.29) we get y = 1. Therefore, the point of intersection of straight lines (2.29) and (2.30) is (1, 1). The straight line (2.28) passes through the point (1, 1) for all values of l. Hence (2.28) passes through the fixed point (1, 1).

Example 2.18 Find the equation of the straight line passing through the intersection of the lines 3x - y = 5 and 2x + 3y = 7 and making an angle of 45° with the positive direction of x-axis.

Solution Solving the equations, 3 x − y = 5 2x + 3y = 7 We get, (2.31) × 3

(2.31) (2.32)

9x - 3y = 15

(2.32)  2x + 3y = 7 On adding, we get 11x = 22.

\ x = 2.

From equation (2.31), 6 - y = 5.

\ y = 1.

Hence (2, 1) is the point of intersection of the lines (2.31) and (2.32). The slope of the required line is m = tanq , m = tan 45° = 1 . Therefore, the equation of the required line is y − y1 = m( x − x1 ) (i.e.) y − 1 = 1( x − 2) ⇒ x − y = 1.

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2-22

Analytical Geometry

Example 2.19 Find the equation of the straight line passing through the intersection of the lines 7x + 3y = 7 and 2x + y = 2 and cutting off equal intercepts on the axes.

Solution The point of intersection of the lines is obtained by solving the following two equations:

7x + 3y = 7

(2.33)



2x + y = 2

(2.34)

(2.33)

7x + 3y = 7

(2.34) × 3

6x + 3y = 6

On subtracting, we get x = 1 and hence y = 0. Therefore, the point of intersection is (1, 0). The equation of the straight line cutting off equal intercepts is x y + = 1 (i.e.) x + y = a. a a This straight line passes through (1, 0). Therefore, 1 + 0 = a (i.e.) a = 1. Hence, the equation of the required straight line is x + y = 1.

Example 2.20 Find the equation of the straight line concurrent with the lines 2x + 3y = 3 and x + 2y = 2 and also concurrent with the lines 3x - y = 1 and x + 5y = 11.

Solution The point of intersection of the lines 2x + 3y = 3 and x + 2y = 2 is obtained by solving the following two equations:

2x + 3y = 3 x + 2y = 2

(2.35)

2x + 3y = 3

(2.36) × 2

2x + 4y = 4

(2.35) (2.36)

On subtracting, we get y = 1 and hence x = 0. Therefore, the point of intersection is (0, 1).

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3 x − y = 1

(2.37)

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The Straight Line



x + 5 y = 11

(2.37) × 5 (2.38) × 1

2-23

(2.38)

15 x − 5 y = 5 x + 5 y = 11

On adding, we get 16x = 16 which gives x = 1 and hence y = 2. The point of intersection of the second pair of lines is (1, 2). The equation of the line joining the two points (0, 1) and (1, 2) is

y −1 1− 2 = = 1 ⇒ y - 1 = x  (i.e.)  x - y + 1 = 0. x − 0 0 −1

Example 2.21 Find the angle between the lines y = 3 x + 4 and y =

1 3

x + 2.

Solution 3. Therefore, m1 = tanq1 = 3 (i.e.) q1 = 60° 1 1 3 (i.e.) q1 = 60°. The slope the line y = x + 2 is . Therefore, 3 3 1 m2 = tan q 2 = (i.e.) q 2 = 30° . The angle between the lines is q1 - q2 = 30°. 3 The slope of the line y = 3 x + 4 is

Example 2.22 Find the equation of the perpendicular bisector of the line joining the points (−2, 6) and (4, -6).

Solution The slope of the line joining the points (−2, 6) and (4, -6) is 1 6+6 −12 m= = = −2. Therefore, the slope of the perpendicular line is . 2 −2 − 4 6  −2 + 4 6 − 6  The midpoint of the line joining the points (−2, 6) and (4, -6) is  ,   2 2  (i.e.) (1, 0). Therefore, the equation of the perpendicular bisector is y - y1 = m (x - x1) 1 (i.e.) y − 0 = ( x − 1) ⇒ 2y = x - 1 or x - 2y - 1 = 0. 2

Example 2.23 A(4, 1), B(7, 4), and C(5, −2) are the vertices of a triangle. Find the equation of the perpendicular line from A to BC.

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2-24

Analytical Geometry

Solution A(4, 1)

B(7, 4)

C(5, −2)

D

The slope of the line BC is m=

y1 − y2 4 + 2 = x1 − x2 7 − 5

6 = 3. 2 1 Therefore, the slope of the perpendicular AD to BC is − . Hence, the equa3 tion of the perpendicular from A(4, 1) on BC is y - y1 = m(x - x1) =



(i.e.) 

−1 ( x − 4) ⇒ 3 y − 3 = − x + 4 3 ∴ x + 3 y = 7. y −1 =

Example 2.24 The foot of the perpendicular from the point (1, 2) on a line is (3,-4). Find the equation of the line.

Solution C (1, 2)

A

D (3, −4)

B

Let AB be the line and D(3, -4) be the foot of the perpendicular from C(1, 2) 2+4 = −3. The slope of the line CD is 1− 3

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The Straight Line

2-25

1 Therefore, the slope of the line AB is . 3 The equation of the line AB is y - y1 = m(x - x1) 1 (i.e.)  y + 4 = ( x − 3) ⇒ 3 y + 12 = x − 3  (i.e.)  x - 3y = 15. 3

Example 2.25 Find the equation of the right bisector of the line joining the points (2, 3) and (4, 5).

Solution The right bisector is the perpendicular bisector of the line joining the points  2 + 4 3 + 5 , (2, 3) and (4, 5). The midpoint of the line is   (i.e.) (3, 4).  2 2  3−5 = 1. 2−4 ∴ The slope of the right bisector is −1. The equation of the right bisector is y - y1 = m(x - x1) ⇒ y - 4 = −1 (x - 3) or y - 4 = -x + 3 or x + y = 7. Therefore, the slope of the given line is m =

Example 2.26 Find the point on the line 3y - 4x + 11 = 0 which is equidistant from the points (3, 2) and (−2, 3).

Solution Let P(x1, y1) be the point on the line 3y - 4x + 11 = 0 which is equidistant from the points A(3, 2) and B(−2, 3). PA2 = ( x1 − 3)2 + ( y1 − 2)2 = x12 + 9 − 6 x1 + y12 + 4 − 4 y1 = x12 + y12 − 6 x1 − 4 y1 + 13 PB 2 = ( x1 + 2)2 + ( y1 − 3)2 = x12 + 4 + 4 x1 + y12 + 9 − 6 y1 = x12 + y12 + 4 x1 − 6 y1 + 13

Since,  PA2 = PB 2 , x12 + y12 − 6 x1 − 4 y1 + 13 = x12 + y12 + 4 x1 − 6 y1 + 13.

(i.e.)  10x1 - 2y1 = 0 or 5x1 - y1 = 0

(2.39)

Since the (x1, y1) lies on the line,

3y - 4x + 11 = 0, 3y1 - 4x1 + 11 = 0

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(2.40)

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2-26

Analytical Geometry

Substituting y1 = 5x1 in (2.40), we get 15x1 - 4x1 + 11 = 0. \ x1 = -1 and hence y1 = -5. Therefore, the required point is (-1, -5).

Example 2.27 Find the equation of the line passing through the point (2, 3) and parallel to 3x - 4y + 5 = 0.

Solution

3 . 4 3 Therefore, the slope of the parallel line is also . 4 Hence the equation of the parallel line through (2, 3) is y - y1 = m(x - x1) 3 (i.e.)  y - 3 = (x - 2) ⇒ 4y - 12 = 3x - 6.  (i.e.)  3x - 4y + 6 = 0. 4 The slope of the line 3x - 4y + 5 = 0 is

Example 2.28 Find the equation of the line passing through the point (4, -5) and is perpendicular to the line 7x + 2y = 15.

Solution

−7 . 2 +2 Therefore, the slope of the perpendicular line is . The equation of the 7 perpendicular line through (4, - 5) is y - y1 = m(x - x1) The slope of the line 7x + 2y = 15 is



(i.e.)

2 ( x − 4) ⇒ 7 y + 35 = 2 x − 8 7 ⇒ 2 x − 7 y = 43.

y+5=



Example 2.29 Find the equation of the line through the intersection of 2x + y = 8 and 3x + 7 = 2y and parallel to 4x + y = 11.

Solution The point of intersection of the lines 2x + y = 8 and 3x + 7 = 2y is obtained by solving the following two equations:

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2x + y = 8

(2.41)

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The Straight Line



3x - 2y = -7

(2.41) × 2

4x + 2y = 16

(2.42)

3 x − 2 y = −7

2-27

(2.42)

9 . 7 −18 38 +8 = . ∴y= 7 7

On adding, we get  7x = 9 ⇒ x =

 9 38  Therefore, the point of intersection is  ,  . 7 7  The slope of the line 4x + y = 11 is -4. The slope of the parallel line is also -4. The equation of the parallel line is y − y1 = m( x − x1 ) ⇒ y −

38 9  = −4  x −  ⇒ 7 y − 38 = −4(7 x − 9)  7 7 (i.e.)  28x + 7y = 74.



Example 2.30 Find the image of the origin on the line 3x - 2y = 13.

Solution O (0, 0)

A

C

B

O ′(x1, y1)

Let O′ (x1, y1) be the image of O on the line AB. Then C is the midpoint of −2 . OO′. The slope of the line OO′ is 3

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2-28

Analytical Geometry

The equation of the line OO′ is y - 0 =

−2 (x - 0) ⇒ 2x + 3y = 0. 3

Solving the equations

3x - 2y = 13

(2.43)

To get the coordinates of C:

2x + 3y = 0

(2.44)

(2.43) × 3

9x - 6y = 39

(2.44) × 2

4x + 6y = 0. On adding 13x = 39. \ x = 3 and hence y = −2.

Therefore, C is (3, −2). C being the midpoint of OO′.

0 + x1 0 + y1 = 3 and = −2.∴x1 = 6 and y1 = -4. 2 2 Therefore, the image is (6, -4).

Example 2.31 Find the equation of the straight line passing through the intersection of the lines 3x + 4y = 17 and 4x - 2y = 8 and perpendicular to 7x + 5y = 12.

Solution 3x + 4y = 17 4x − 2 y = 8



(2.45) (2.46)

(2.45) × 1 + (2.46) × 2 gives 3 x + 4 y = 17 4 x − 2 y = 16



11x = 33 ⇒ x = 3. From (2.45), 9 + 4y = 17. Therefore, y = 2. Hence (3, 2) is the point of inter−7 section of the lines (2.45) and (2.46). The slope of the line 7x + 5y = 12 is . 5 5 Therefore, the slope of the perpendicular line is y − 2 = ( x − 3). 7 7y - 14 = 5x - 15  or  5x - 7y = 1.

Example 2.32 Find the orthocentre of the triangle whose vertices are (5, −2), (–1, 2), and (1, 4).

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The Straight Line

2-29

Solution A(5, −2)

E

B(−1, 2)

D

C(1, 4)

2−4 = 1. Therefore, slope of the perpendicular AD is −1. −1 − 1 The equation of the line AD is y + 2 = −1(x − 5).

Slope of BC =

y + 2 = -x + 5 or x + y = 3



(2.47)

4 + 2 −3 2 = . Therefore, slope of BE is . 1− 5 2 3 The equation of BE is

Slope of AC is



y−2 =

2 ( x + 1)  or  3y - 6 = 2x + 2 or 2x - 3y + 8 = 0 3

(2.48)

Solving the equations (2.47) and (2.48), we get the coordinates of the orthocentre: (2.47) × 3

3x + 3y = 9

(2.48) × 1

2 x − 3 y = −8

1 On adding 5x = 1 or x = . 5 1 14 From (2.47), y = 3 − = . 5 5  1 14  \ The orthocentre is  ,  . 5 5 

Example 2.33 The points (1, 3) and (5, 1) are two opposite vertices of a rectangle. The other two vertices lie on the line y = 2x + c. Find c and the remaining two vertices.

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2-30

Analytical Geometry

Solution D

C(5, 1) y=

2x

+c

E

A(1, 3)

B(x, 2x − 4)

Let ABCD be the rectangle with A and C as the points with coordinates (1, 3) and (5, 1), respectively. In a rectangle the diagonals bisect each other.  1 + 5 3 + 1 The midpoint AC is  ,  (i.e.) (3, 2).  2 2  As this point lies on BD whose equation is y = 2x + c. We get 2 = 6 + c or c = -4. Therefore, the equation of the line BD is y = 2x − 4. Therefore, the coordinates of any point on this line is (x, 2x − 4). If this is the point B then AB 2 + BC 2 = AC 2.

(1 − x )2 + (3 − 2 x + 4)2 + ( x − 5)2 + (2 x − 4 − 1)2 = (1 − 5)2 + (3 − 1)2



(i.e.) 

(1 − x )2 + (7 − 2 x )2 + ( x − 5)2 + (2 x − 5)2 = 16 + 4 ⇒ 10 x 2 − 60 x + 30 = 0 ⇒ x = 2, 4.

As y = 2 x − 4, the corresponding values of y = 0, 4. Therefore, the coordinates of B and D are (2, 0) and (4, 4).

Example 2.34 If a, b, and c are distinct numbers different from 1 then show that the points  a3 a 2 − 3   b3 b 2 − 3   c3 c2 − 3 , , , , and A B C  c − 1 , c − 1  are collinear if  a − 1 a − 1   b − 1 b − 1  ab + bc + ca - abc = 3(a + b + c).

Solution Let A, B, and C lie on the straight line px + qy + r = 0. Then the equation of the line satisfies the condition  t3   t 2 − 3 3 2 + p q  t − 1  + r = 0 where t = a, b, and c (i.e.) pt + qt + rt −  t − 1 3q - r = 0. Here, a, b, and c are the roots of this equation.

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The Straight Line

−q 3q + r r , ab + bc + ca = , abc = p p p

a+b+c = ∴ abc − 3

2-31

q r − = 0. p p

or abc + 3( a + b + c) − ( ab + bc + ca) = 0. ∴ ab + bc + ca − abc = 3( a + b + c).



Example 2.35 A vertex of an equilateral triangle is at (2, 3) and the equation of the opposite side is x + y = 2. Find the equations of the other sides.

Solution A(2, 3)

p/3 x+y=2

B

C

The slope of BC is −1. Let m be the slope of AB or AC. Then tan

(i.e.)  ∴m =

3 −1 3 +1

or

3 (1 − m) = ±( m + 1) 3 +1 3 −1

p ( m + 1) =± 3 1− m

= 2 − 3 or 2 + 3.

Therefore, the equation of other two sides are ( x − 2) and y − 3 = (2 + 3 )( x − 2).

y − 3 = (2 + 3 )

(i.e.)  (2 − 3 ) x − y + 2 3 − 1 = 0   and  (2 + 3 ) x − y − (2 3 + 1) = 0.

(2 + 3 ) x − y + 3 − 2(2 + 3 = 0) (2 − 3 ) x − 2(2 − 3 ) − y + 3 = 0



( 2 + 3 ) x − y − 2 3 − 1 = 0)

( 2 − 3 x − y − 1 + 2 3 = 0)

Example 2.36

x y + = 1 intercepted a b between the axes. Find the equation of the other diagonal.

One diagonal of a square is the portion of the line

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2-32

Analytical Geometry

Solution The slope of AB is −

b . Let m be slope of AC. a

b

O

45 45 A

D a

b   m +  p a  am + b  ⇒1= ± ∴ tan = ±  a − mb  b 4   1 − m   a





C ( a, b 2 2 )

B

∴m =

a−b a+b

or

a+b  a b . The midpoint of AB is  ,  .  2 2 a−b

The equation of the side OC is

y−

b a−b = x 2 a+b

(2.49)

b  a + b = − x  a − b  2

(2.50)

The equation of the side BD is

y−

The equation of the other diagonal is

y−

b a a =  x −  2 b 2

(2.51)

Example 2.37 If the vertices of DABC are (xi, yi) i = x y median through A is given by x1 y1 x2 y2

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1, 2, 3. Show that the equation of the 1 x y 1 1 + x1 y1 1 = 0. 1 x3 y3 1

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The Straight Line

2-33

Solution

 x + x3 y2 + y3  The coordinates of the midpoint of BC are D  2 , .  2 2 

A(x1, y1)

G

B(x2, y2)

D

C(x3, y3)

x The equation of the median AD is given by x1 the area of a line is zero. x2 + x3 2



 (i.e.) 



x x1 x2

\ 

x x1 x2 + x3

y y1 y2 + y3 2

1 1 = 0. Since 1

y 1 y1 1=0 y2 + y3 1

y 1 x y1 1 + x1 y2 1 x3

y 1 y1 1 = 0. y3 1

Example 2.38 If (x, y) is an arbitrary point on the internal bisector of vertical angle A of DABC, where (xi, yi), i = 1, 2, 3 are the vertices of A, B, and C, respectively, and a, b, and c are the length of the sides BC, CA, and AB, respectively, prove x y 1 x y 1 that b x1 y1 1 + c x1 y1 1 = 0. x2 y2 1 x3 y3 1

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2-34

Analytical Geometry

Solution In DABC, AD is the internal bisector of A. We know that A(x1, y1)

c

b

c B(x2, y2)

b D a

C(x3, y3)

BD AB BD c = ,∴ = . DC AC DC b



 bx + cx3 by2 + cy3  , The coordinates of D are  2  b+c b + c  x y The equation of AD is given by x1 y1 bx2 + cx3 by2 + cy3 b+c b+c x y 1 (i.e.)  x1 y1 1 = 0. bx2 + cx3 by2 + cy3 b + c



x (i.e.)  x1 bx2 x \  b x1 x2

y y1 by2

1 x 1 + x1 b cx3

y 1 x y1 1 + c x1 y2 1 x3

y y1 cy3

1 1=0 1

1 1 = 0. c

y 1 y1 1 = 0. y3 1

Example 2.39 Find the orthocentre of the triangle whose vertices are (a, 0), (0, b), and (0, 0).

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The Straight Line

2-35

Solution B(0, b)

A(a, 0) O

The orthocentre is the point of concurrence of altitudes. Since OA and OB are perpendicular to each other, OA and OB are the altitudes through A and B of DABC. Therefore, 0 is the orthocentre. Hence, the coordinates of the orthocentre is (0, 0).

Example 2.40 Prove that the orthocentre of the triangle formed by the three lines a a a y = xt1 + , y = xt2 + , and y = xt3 + lies on the line x + a = 0. t1 t2 t3

Solution The equation of the line passing through the intersection of the lines A E

F H B

D

C

a y = xt1 + t1



a a and y = xt3 + is t2 t3   a a  y − xt2 − t  + k  y − xt3 − t  = 0. 2 3 y = xt2 +

t2 + kt3 . The slope of the line BC is t1. Since AD is 1+ k t + kt3 perpendicular to BC, 2 × t1 = −1 k +1

The slope of the line AD is

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2-36

Analytical Geometry

⇒ t1t2 + kt1t3 = − k − 1



∴k =



The equation of   a a xt2 −  − (t1t2 + 1)  y − xt3 −  = 0. t  t   2

the

line

− (t1t2 + 1) . t1t3 + 1

AD

is

  a a (t1t3 + 1)  y − xt2 −  − (t1t2 + 1)  y − xt3 −  = 0 t2  t3   

3

(i.e.)  t1 t2 t3 y + t2 t3 x − a(1 + t1t3 + t1t2 ) = 0.



(2.52)

Similarly the equation of the line BE is t1t2 t3 y + t3t1 x − a (1 + t1t2 + t2 t3 ) = 0



(2.53)

Subtracting equations (2.52) from (2.53), t3 (t1 - t2) x + at3(t1 - t2) = 0. Since t1 ≠ t2, x + a = 0 the orthocentre lies on the line x + a = 0.

Example 2.41 Show that the reflection of the line px + qy + r = 0, on the line lx + my + n = 0 is ( px + qy + r )(l 2 + m2 ) − 2 (lp + mq) (lx + my + n) = 0.

Solution

px

r y+

+q

B

=0 L

P lx + my + n = 0

A

C

M D

Let AD be the reflection of the line px + qy + r = 0 in the line lx + my + n = 0. Then the equation of line AD is px + qy + r + k (lx + my + n) = 0. Then the perpendicular from any point on AC to AB and AD are equal.

∴

px + qy + r p +q 2

2

=±

( px + qy + r ) + k (lx + my + n) ( p + kl )2 + (q + km)2

Since the point P lies on AC, lx + my + n = 0, lx1 + my1 + n = 0

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The Straight Line

∴ p 2 + q 2 = ( p + kl )2 + (q + km)2

p2 + q2 = p2 + q2 + k 2 (l 2 + m2 ) + 2k ( pl + qm) −2 ( pl + qm) ∴ k (l 2 + m2 ) + 2 ( pl + qm) = 0 ⇒ k = l 2 + m2

Hence the equation of the 2 ( pl + qm) (lx + my + n) = 0.

line

AD

is

(l 2 + m2 ) ( px + qy + r ) −

Example 2.42 The diagonals of the parallelogram are given by the sides u = p, u = q, v = r, v = s where u = ax + by + c and v = a1x + b1y + c1. Show that the equation of the diagonal which passes through the points of intersection of u v 1 u = p, v = r and u = q and r = s is given by p r 1 = 0. q s 1

Solution Consider

u v 1 p r 1=0 q s 1 u=q

C

s

v=

v=

a1 x

+b

1y

v=

r

+c

1

=0

D

(2.54)

A

u=p

B

u = ax + by + c = 0

This is a linear equation in x and y and, therefore, it represents a straight line. The coordinates of B are given by the intersection of the lines u = p and v = s. However, u = p and v = r satisfies the equation (2.54). In addition, u = q, and v = s satisfy the equation (2.54) and hence the equation (2.54) is the line ­passing through B and D and represents the equation of the diagonal BD.

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Analytical Geometry

Example 2.43 A line through the point A(-5, -4) meets the lines x + 3y + 2 = 0, 2x + y + 4 = 0, and x - y - 5 = 0 at the points B, C, and D, respectively. If 2

2

2

 15   10   6    +   =   find the equation. AB AC AD 

Solution The equation of the line passing through the point (-5, -4) is x+5 x+4 = = r cos q sin q



(2.55)

Any point on the line is (rcos q - 5, rsin q - 4). The point meets the line x + 3y + 2 = 0 at B then AB = r ⋅ (rcos q - 5) + 3 (rsin q - 4) + 2 = 0. r(cos q + 3sin q ) = 15



∴r =

15 cos q + 3 sin q

or

15 = cos q + 3 sin q AB

(2.56)

If the line (2.55) meets the line 2x + y + 4 = 0 at C then 2 ( r cos q − 5) + ( r sin q − 4) + 4 = 0. Now, r = AC and 10 = 2 cos q + sin q AC



(2.57)

If the line (2.55) meets the line x - y - 5 = 0 then (rcos q - 5) - (rsin q - 4) 5 = 0 and here AD = r. ∴

2

2

6 = (cos q − sinq ) AD

 15   10   6  Given that  + =  AB   AC   AD 

2



(i.e.)  (cos q + 3 sin q )2 + (2 cos q + sin q )2 = (cosq − sin q )2



−2 2 ∴ 2 cos q + 3 sin q = 0 or tan q = − . ∴ m = . 3 3

Hence, the equation of the line is y + 4 =

M02_ANALYTICALGEOMETRY_ISBN_CH02.indd 38

(2.58)

−2 ( x + 5) or 2 x + 3 y + 22 = 0. 3

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The Straight Line

2-39

Example 2.44 A variable straight line is drawn through O to cut two fixed lines L1 and L2 at l+q p q = + A1 and A2. A point A is taken on the variable line such that . OA OA1 OA2 Show that the locus of P is a straight line passing through the point of intersection of L1 and L2.

Solution r2 r

A

A1

X A2 L2

O L1

x y = = r where OA = r . Any cos q sin q point on this line is (rcos q, rsin q ). Let OA1 = r1 and OA2 = r2. A1 is (r1cos q, r1sin q ) and A2 is (r2cos q, r2sin q ). Let the two fixed straight lines be L1: l1x + m1y - 1 = 0 and L2: l2x + m2y - 1 = 0. Since the points A1 and A2 lie on the two lines, respectively, Let the equation of the line OX be



l1 ( r1 cos q ) + m1 ( r1 sin q ) = 1.



l2 ( r2 cos q ) + m2 ( r2 sin q ) = 1 ∴



1 = l1 cos q + m1 sin q r1 1 = l2 cos q + m2 sin q r2 p q p+q + = OA1 OA2 OA





p+q r p (l1r cos q + m1r sin q ) + q (l2 r cos q + m2 r sin q ) = p + q



(i.e.)  p (l1 x + m1 y ) + q (l2 x + m2 y ) − ( p + q) = 0



(i.e.)  p (l1 cos q + m1 sin q ) + q (l2 cos q + m2 sin q ) =

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2-40

Analytical Geometry

(i.e.) p (l1 x + m1 y − 1) + q (l2 x + m2 y − 1) = 0 which is a straight line passing through the point of intersection of the two fixed straight lines L1 = 0 and L2 = 0.

Example 2.45 If the image of the point (x1, y1) with respect to the line my + lx + n = 0 is the x −x y − y1 −2 (lx1 + my1 + n) = point (x2, y2) show that 2 1 = 2 . l m l 2 + m2

Solution P(x1, y1)

lx + my + n = 0

R

Q (x2, y2)

Q(x2, y2) is the reflection of P(x1, y1) on the line lx + my + n = 0. The midpoint y − y2 . The slope of PQ lies on the line lx + my + n = 0. The slope of PQ is 1 x1 − x2 −l of the line is lx + my + n = 0 is . Since these two lines are perpendicular, m



 y1 − y2   −l   x − x   m  = −1 or l ( y1 − y2 ) = m ( x1 − x2 ) 1 2 or 

=



∴

( x1 − x2 )2 + ( y1 − y2 )2

x1 − x2 y1 − y2 = = l m 2 PR l +m 2

2

l +m 2

=

2 l +m 2

2

⋅

2

lx1 + my1 + n l +m 2

2

PQ

=

l + m2 2

=

2(lx1 + my1 + n) l 2 + m2

x2 − x1 y2 − y1 −2(lx1 + my1 + n) = = . l m l 2 + m2

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The Straight Line

2-41

Example 2.46 Prove that the area of the triangle whose roots are Lr = arx + br y + cr (r = 1, ∆2 where Ci is the cofactor of ci (i = 1, 2, 3) in D given by 2, 3) is 2C1C2C3 a1 b1 c1 ∆ = a2 b2 c2 . a3 b3 c3

Solution Let Ar, Br, and Cr be the cofactors of ar, br, and cr in D. The point of intersection  bc −b c c a −c a  of the lines a1 x + b1 y + c1 = 0, a2 x + b2 y + c2 = 0 is  1 2 2 1 , 1 2 2 1   a1b2 − a2 b1 a1b2 − a2 b1  A B  (i.e.)   3 , 3   C3 C3 



A

c2

+b

3y

y+ b2

+c

3

=0

x+ a2

a

3x

=0

B

C

a1x + b1y + c1 = 0

A B  A B A B  \ The vertices of the triangle are  1 , 1  ,  2 , 2  , and  3 , 3  . Then  C1 C1   C2 C2   C3 C3  the area of the triangle is given by,



M02_ANALYTICALGEOMETRY_ISBN_CH02.indd 41

∆1 =

A1 C1

B1 C1

1

1 A2 2 C2

B2 C2

1

A3 C3

B3 C3

1

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2-42

Analytical Geometry

where D is the determinant formed by the cofactors.

=

D ∆2 =   (Since we have the property D = ∆2). 2C1C2C3 2C1C2C3

Example 2.47 A straight line L intersects the sides BC, CA, and AB of a triangle ABC in D, BD CE AF E, and F, respectively. Show that ⋅ ⋅ = −1. DC EA FB

Solution Let DEF be the straight line meeting BC, CA, and AB at D, E, and F, respectively. Let the equations of the line DEF be lx + my + n = 0. A F

E

D

B C

Let D divide BC in the ratio l:1. Then the coordinates of D are  l x3 + x2 l y3 + y1   l + 1 , l + 1  . As this point lies on the line lx + my + n = 0.

 l y + y2   l x + x2  + m 3 l 3 +n=0  l + 1   l + 1 



⇒ l ( l x3 + x2 ) + m ( l y3 + y2 ) + n ( l + 1) = 0



(i.e.)  l (lx3 + my3 + n) + lx2 + my2 + n = 0 ∴l = −





∴

M02_ANALYTICALGEOMETRY_ISBN_CH02.indd 42

(lx2 + my2 + n) lx3 + my3 + n

(lx + my2 + n) BD =− 2 DC lx3 + my3 + n

4/22/2013 8:43:06 PM

The Straight Line

Similarly

2-43

(lx + my3 + n) AF (lx + my1 + n) CE = −1 1 =− 3 ; EA lx1 + my1 + n FB lx2 + my2 + n

Multiplying these three we get

BD CE AF × × = −1. DC EA FB

Example 2.48 A straight line is such that the algebraic sum of perpendiculars drawn upon it from any number of fixed points is zero. Show that the straight line passes through a fixed point.

Solution Let (x1, y1), (x2, y2),…, (xn, yn), be n fixed points and ax + by + c = 0 be a given line. The algebraic sum of the perpendiculars from (xi, yi), i = 1, 2, …, n to this line is zero. n ax + byi + c ∴∑ i =0 i =1 a2 + b2

a Σxi bΣyi nc = 0. Where n = a 2 + b 2 + + n n n

 x + x +  + xn y1 + y2 +  + yn  , This equation shows that the point  1 2  lies  n n on the line ax + by + c = 0. Therefore, the line passes through a fixed point.

Example 2.49 Determine all the values of a  for which the point (a, a 2) lies inside the triangle formed by the lines 2x + 3y - 1 = 0, x + 2y - 3 = 0, and 5x - 6y - 1 = 0.

Solution

L2

=0

A

L

3

=0 (a , a 2)

B

M02_ANALYTICALGEOMETRY_ISBN_CH02.indd 43

L1 = 0

C

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2-44

Analytical Geometry

L1: 2x + 3y – 1 = 0 L2: x + 2y – 3 = 0 L3: 5x – 6y – 1 = 0.

2x + 3y − 1 = 0 2x + 4 y − 6 = 0

5 x − 10 y − 15 = 0 5x − 6 y − 1 = 0

4x + 6 y − 2 = 0 5x − 6 y − 1 = 0

y=5

16 y = 14

9x = 3

x = -7

y=

7 8

x=

1 3

\C(-7, 5)

x=

5 4

y=

1 4

 5 7 ∴A=  ,   4 8



 1 1 Hence B is  ,   3 8

 5  7 L1 (A): 2   + 3   − 1 > 0.  4  8

\ L1 (a , a 2) = 2a + 3a 2 - 1 > 0 if points A and (a, a 2) lies on the same side of the line. 3a 2 + 2a - 1 > 0 ⇒ (3a  - 1) (a  + 1) > 0.

1 a < −1 or a > 3 1 2 L2 ( B) = + − 3 < 0 ; 3 3 2 2 \ L2(a, a  ) = a  + 2a  – 3 < 0 (i.e.)  = 2a 2 + a  - 3 < 0 −3 < a < 1 2



= (2a  + 3) (a  - 1) < 0  ∴



L3(-7, 5) = −35 - 30 − 1 < 0 ∴ L3 (a , a 2 ) = 5a − 6a 2 − 1 < 0 ⇒ 6a 2 − 5a + 1 > 0



(I)

⇒ (3a − 1)(2a − 1) > 0 ⇒ a
3 2

(II)

(III)

 −3   1  From the conditions I, II, and III, we have a ∈  , −1 ∪  ,1  2  2 

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2-45

The Straight Line

Example 2.50 Find the direction in which a straight line must be drawn through the point (1, 4) so that its point of intersection with the line x + y + 5 = 0 may be at a distance 5 2 units.

Solution

x −1 y − 4 = = r. cos q sin q Any point on this line is ( r cos q + 1, r sin q + 4). If this point lies on the line x + y + 5 = 0 then r cos q + 1 + r sin q + 4 + 5 = 0. Let the equation of the line through the point (1, 4) be



r (cos q + sin q ) = −10. If r = 5 2 then cos q + sin q = − 1



2 cos

cos q +

1 2

10 5 2

=− 2

sin q = −1

p p cos q + sin sin q = −1 4 4 p  cos q −  = cos p  4 ∴q =



p 5p + p or q = . 4 4

5p with the positive direc\ The required straight line makes an angle of tions of x-axis and passes through the point (1, 4). 4

Exercises 1.

Find the area of triangle formed by the axes, the straight line L passing through the points (1, 1) and (2, 0) and the line perpendicular to the L and 1  passing through  , 0 . 2 

2.

Ans.:

25 sq. units 16

The line 3x + 2y = 24 meets y-axis at A and x-axis at B. The perpendicular bisector of AB meets the line through (0, −1) parallel to x-axis at C. Find the area DABC. Ans.: 91 sq. units

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2-46

Analytical Geometry

3.

If (x, y) be an arbitrary point on the altitude through A of DABC with vertices (xi, yi), i = 1, 2, 3 then the equation of the altitude through A is x y 1 x y 1 b sec B x1 y1 1 + c sec C x1 y1 1 = 0. x2 y2 1 x 2 y2 1

4.

A ray of light is sent along the line x - 2y − 3 = 0. Upon reaching the line 3x - 2y - 5 = 0 the ray is reflected from it. Find the equation of the line containing the reflected ray. Ans.: 29x - 2y - 31 = 0

5.

The extremities of the diagonals of a square are (1, 1) and (−2, −1). Obtain the equation of the other diagonal. Ans.: 6x + 4y + 3 = 0

6.

The straight line 3x + 4y = 5 and 4x - 3y = 15 intersect at the point A. On this line, the points B and C are chosen so that AO = AC. Find the possible equations of the line BC passing through the point (1, 2). Ans.: x - 7y + 13 = 0 and 7x + y - 9 = 0

7.

The consecutive sides of a parallelogram are 4x + 5y = 0 and 7x + 2y = 0. If the equation of one diagonal is 11x + 7y = 9, find the equation of the other diagonal. Ans.: x - y = 0

8.

Show that the lines ax ± by ± c = 0 enclose a rhombus of area

9.

If the vertices of a ∆OBC are O(0, 0), B(−3, −1), and C(−1, −3), find the equation of the line parallel to BC and intersecting sides OB and OC whose 1 perpendicular distance from (0, 0) is . 2 Ans.: 2 x + 2 y + 2 = 0

10.

Find the locus of the foot of the perpendicular from the origin upon the line joining the points ( a cosq , b sinq ) and ( −a sinq , b cosq ) where a is a variable. Ans.: a2x2 + b2y2 = 2(x2 + y2)2

11.

Show that the locus given by x + y = 0, (a - b)x + (a + b)y = 2ab and (a + b)x +

2c 2 . ab

 a (a - b)y = 2ab form an isosceles triangle whose vertical angle is 2 tan −1   .  b Determine the centroid of a triangle. b Ans.:  , 3 12.

b  3

The sides of a quadrilateral have the equations, x + 2y = 3, x = 1, x - 3y = 4, and 5x + y + 12 = 0. Show that the diagonals of the quadrilateral are at right angles.

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2-47

The Straight Line

13.

Given n straight lines and a fixed point O. Through O a straight line is drawn meeting these lines in the point A1 , A2 ,…, An and a point A such that 1 1 1 n = + ++ . Prove that the locus of the point A is a OA OA1 OA2 OAn straight line.

14.

Find the equation of the line joining the point (3, 5) to the point of intersection of the lines 4x + y - 1 = 0 and 7x - 3y - 35 = 0 and prove that the line is equidistant from the origin and the points A, B, C, and D.

15.

Find the equation of the line passing through the point (2, 3) and making intercepts of length 2 units and between the lines. Ans.: 3x + 4y - 8 = 0 and x - 2 = 0

16.

If x cos a + y sin a = p where p =

sin 2 a be a straight line, prove that the cos a perpendiculars p1, p2, and p3 on the line from the point (m2, 2m), (mm′, m + m′), and (m′2, 2m′), respectively, are in G.P.

17.

Prove that the points (a, b), (c, d), and (a - c, b - d) are collinear if (ad = bc). Also, show that the straight line passing through these points passes through the origin.

18.

One diagonal of a square is along the line 8x - 15y = 0 and one of its vertices is (1, 2). Find the equations of the sides of the square through this vertex. Ans.: 2x + y = 4 and x - 2y + 3 = 0

19.

Find the orthocentre of a triangle formed by lines whose equations are x + y = 1, 2x + 8y = 6, and 4x - y + 4 = 0. −3 22 Ans.:  ,   7 7

20.

The sides of a triangle are ur = x cos ar + y sin ar - pr = 0, r = 1, 2, 3. Show that its orthocentre is given by u1 cos(a2 - a3) = u2 cos(a3 - a1) = u3 cos(a1 - a2).

21.

Find the equation of straight lines passing through the point (2, 3) and having an intercept of length 2 units between the straight lines 2x + 3y = 3 and 2x + y = 5. Ans.: x = 2, 3x + 4y = 18

22.

Let a line L has intercepts a and b on the coordinate axes. When the axes are rotated through an angle, keeping the origin fixed, the same line L has intercepts p and q. Obtain the relation between a, b, p, and q. Ans.:

23.

1 1 1 1 + = + p2 q2 a2 b2

A line through the variable point A(k + 1, 2k) meets the line 7x + y - 16 = 0, 5x - y + 8 = 0, x - 5y + 8 = 0 at B, C, and D, respectively. Prove that AC, AB, and AD are in G.P.

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2-48

Analytical Geometry

24.

Find the equation of the straight lines passing through (−2, -7) and having an intercept of length 3 between the straight lines 4x + 3y = 12 and 4x + 3y = 3. Ans.: x + 2 = 0, 7x - 24y + 182 = 0

25.

A line is such that its segment between the straight lines 5x - y - 4 = 0 and 3x + 4y - 4 = 0 is bisected at the point (1, 5). Obtain its equation. Ans.: 83x - 35y + 92 = 0.

26.

Prove that the (a - b) x + (b - c)y + (c - a) = 0, (a - c)x + (c - a)y + (a - b) = 0, and (c - a) x + (a - b)y + (b - c) = 0 are concurrent.

27.

Two vertices of a triangle are (5, −1) and (−2, 3). If the orthocentre of the triangle is at the origin, find the coordinates of the third vertex. Ans.: (-4, -7)

28.

A line intersects x-axis at A(7, 0) and y-axis at B(0, -5). A variable line PQ which is perpendicular to AB intersects x-axis at P and y-axis at Q. If AQ and BP intersect at R, then find the locus of R. Ans.: x2 + y2 – 7x + 5y = 0

29.

A rectangle PQRS has its side PQ parallel to the line y = mx and vertices P, Q, and S on the lines y = a, x = b, and x = −b, respectively. Find the locus of the vertex R. Ans.: (m2 - 1)x - my + b(m2 + 1) + am = 0

30.

Determine the condition to be imposed on b  so that (O, b ) should be on or inside the triangle having sides y + 3x + 2 = 0, 3y - 2x - 5 = 0, and 4y + x - 14 = 0. 5 7 Ans.: ≤ b ≤ 3 2

31.

Show that the straight lines 7x - 2y + 10 = 0, 7x + 2y - 10 = 0, and y = 2 form an isosceles triangle and find its area. Ans.: 14 sq. units

32.

The equations of the sides BC, CA, and AB of a triangle ABC are Kr = ar x + br  y + cr = 0, r = 1, 2, 3. Prove that the equation of a line drawn through A parallel to BC is K3(a2b1 - a1b2) = K2(a3b1 - a1b3).

33.

The sides of a triangle ABC are determined by the equation ur = arx + bry + cr = 0, r = 1, 2, 3. Show that the coordinates of the orthocentre of the triangle ABC satisfy the equation l1u1 = l2u2 + l3u3 where l1 = a2a3 + b2b3, l2 = a3a1 + b3b1, and l3 = a1a2 + b1b2.

34.

Prove that the two lines can be drawn through the point P(P, Q) so that their perpendicular distances from the point Q(2a, 2a) will be equal to a and find their equations. Ans.: y = a, 4x - 3y + 3a = 0.

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2-49

The Straight Line

35.



Find the locus of a point which moves such that the square of its distance from the base of an isosceles triangle is equal to the rectangle under its distances from the other sides. 2  y  1 1  l2  Ans.:  y 2  2 + 2   = ±  − 1 − 2  a  b   a  b 

36.

Prove that the lines given by (b + c)x – bcy = a(b2 + bc + c2), (c + a)x – cay = b(c2 + ca + a2), and (a + b)x – aby = c(a2 + ab + b2) are concurrent.

37.

Show that the area of the triangle formed by the lines y = m1x + c1, y = m2x + 1  ( c − c ) 2 ( c − c )3 ( c − c ) 2  c2, and y = m3x + c3 is  2 3 + 3 1 + 1 2  . m3 − m1 m1 − m2  2  m2 − m3

38.

Find the bisector of the acute angle between the lines 3x + 4y = 1 which is the bisector containing the origin. Ans.: 11x + 3y - 17 = 0 (origin lies in the obtuse angle between the lines.)

39.

If a1a2 + b1b2 > 0 prove that the origin lies at the obtuse angle between the lines a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0, where c1 and c2 both being of the same sign.

40.

Find the equation to the diagonals of the parallelogram formed by the lines ax + by + c = 0 , ax + by + d = 0, a′x + b′y + c′ = 0, a′x + b′y − d ′ = 0 . Show that the parallelogram will be a rhombus if 2 2 ( a 2 + b 2)(c′ − d ′)2 = ( a′ + b′ )(c − d )2 .

41.

A variable line is at a constant distance p from the origin and meets coordinate axes in A and B. Show that the locus of the centroid of the ∆OAB is x −2 + y −2 = p −2 .

42.

A moving line is lx + my + n = 0 where l, m, and n are connected by the relation al + bm + cn = 0, and a, b, and c are constants. Show that the line passes through a fixed point.

43.

Find the equation of bisector of acute angle between the lines 3x – 4y + 7 = 0 and 12x + 5y - 2 = 0.

44.

Q is any point on the line x - a = 0 and O is the origin. If A is the point (a, 0) and QR, the bisector OQA meets x-axis on R. Show that the locus of the foot of the perpendicular from R to OQ is the (x – 2a) (x2 + y2 + a2x) = 0.

45.

The lines ax + by + c = 0, bx + cy + a = 0, and cx + ay + b = 0 are concurrent where a, b, and c are the sides of the DABC in usual notation and prove that sin 3 A + sin 3 B + sin 3 C = 3 sin A sin B sin C .

46.

A variable straight line OPQ passes through the fixed point O, meeting the two fixed lines in points P and Q. In the straight line OPQ, a point R is taken such that OP, OR, and OQ are in harmonic progression. Show that the locus of point Q is a straight line.

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2-50

47.

Analytical Geometry

A ray of light is set along the line x – 2y – 3 = 0. On reaching the line 3x – 2y – 5 = 0, the ray is reflected from it. Find the equation of the line containing the reflected ray 2qx – 2y – 31 = 0. Ans.: 2qx - 2y - 31 = 0.

 48.

Let DABC be a triangle with AB = AC. If D is the midpoint of BC, and E is the foot of the perpendicular drawn from D to AC and F is the midpoint of BE. Prove that AF is perpendicular to BE. Ans.: 14x + 23y - 40 = 0.

49.

The perpendicular bisectors of the sides AB and AC of a triangle ABC are x – y + 5 = 0 and x + 2y = 0, respectively. If the point A is (1, −2), find the equation of the line 14x + 23y – 40 = 0.

50.

A triangle is formed by the lines ax + by + c = 0, lx + my + n = 0, and px + ax + by + c lx + my + n passes qy + r = 0. Show that the straight line = cp + bq lp + mq through the orthocentre of the triangle.

51.

Prove that the diagonals of the parallelogram formed by the lines ax + by + c = 0, ax + by + c′ = 0, a′ x + b′ y + c = 0, and a′ x + b′ y + c′ = 0 will 2 2 be at right angles if a 2 + b 2 = a′ + b′ .

52.

One diagonal of a square is the portion of the line

x y + = 1 intercepted a b between the axes. Show that the extremities of the other diagonal are  a + b a + b  and  a − b a − b  . , ,     2 2  2 2 

53.

Show that the origin lies inside a triangle whose vertices are given by the equations 7x – 5y – 11 = 0, 8x + 3y + 31 = 0, and x + 3y – 19 = 0.

54.

A ray of light travelling along the line OA, O being the origin, is reflected by the line mirror x – y + 1 = 0, the point of incidence A is (1, 2). The reflected ray is again reflected by the mirror x - y = 1, the point of incidence being B. If the reflected ray moves along BC, find the equation of BC. Ans.: 2x - y - 6 = 0

55.

If the lines p1x + q1y = 1, p2x + q2y = 1, and p3x + q3y = 1 are concurrent, prove that the points (p1, q1), (p2, q2), and (p3, q3) are collinear.

56.

If p, q, and r be the length of the perpendiculars from the vertices A, B, and C of a triangle on any straight line, prove that a 2 ( p − q)( p − r ) + b 2 (q − r )(q − p) + c 2 ( r − p)( r − q) = 4 ∆ 2 .

57.

Prove that the area of the parallelogram formed by the straight line a1x + b1y + c1 = 0, a1 x + b1 y + d1 = 0, a2 x + b2 y + c2 = 0, and a2 x + b2 y + d2 = 0 is ( d1 − c1)( d2 − c2) . a1b2 − a2b1

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The Straight Line

2-51

58.

A ray of light is sent along the line 2x - 3y = 5. After refracting across the line x + y = 1, it enters the opposite sides after turning by 15° away from the line x + y = 1. Find the equation of the line along which the refracted ray travels   Ans.: (15 3 − 20) x − (30 − 10 3 ) y + (11 − 18 3 ) = 0.

59.

Two sides of an isosceles triangle are given by the equations 7x – y + 3 = 0 and x + y − 7 = 0 and its third side passes through the point (1, -10). Determine the equation of the third side. Ans.: x − 3 y − 31 = 0, 3 x + y + 7 = 0.

60.

Find all those points on the line x + y = 4 which are at c unit distance from the line 4x + 3y = 10.

61.

Are the points (3, 4) and (2, -6) on the same or opposite sides of the line 3x - 4y = 8? Ans.: opposite sides

62.

How many circles can be drawn each touching all the three lines x + y = 1, y = x, and 7x - y = 6? Find the centre and radius of one of the circles. 7 T  . Ans.: Focus: (0, 7) Incentre  , 1 , r =  12  12 2

63.

t t   Show that P 1 + ,2+  be any point on a line then the range of values  2 2 of t for which the point p lies between the parallel lines x + 2y = 1 and 2x +  −4 2 5 2  4y = 15 is  , . 6   5

64.

Show that a, b, and c are any three terms of AP then the line ax + by + c = 0 always passes through a fixed point.

65.

Show that if a, b, and c are in G.P., then the line ax + by + c = 0 forms a triangle with the axes, whose area is a constant.

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Chapter

3

Pair of Straight Lines

3.1  INTRODUCTION We know that every linear equation in x and y represents a straight line. That is Ax + By + C = 0, where A, B and C are constants, represents a straight line. Consider two straight lines represented by the following equations:

l1 x + m1 y + n1 = 0

(3.1)



l2 x + m2 y + n2 = 0

(3.2)

(l1 x + m1 y + n1 ) (l2 x + m2 y + n2 ) = 0   

(3.3)

Also consider the equation

If (x1, y1) is a point on the straight line given by (3.1) then l1 x1 + m1 y + n1 = 0



This shows that (x1, y1) is also a point on the locus of (3.3). Therefore, every point on the line given by (3.1) is also a point on the locus of (3.3). Similarly, every point on the line given by (3.2) is also a point on the locus of (3.3). Therefore, (3.3) satisfies all points on the straight lines given by (3.1) and (3.2). Hence, we say (3.3) represents the combined equation of the straight lines given by (3.1) and (3.2). It is possible to rewrite (3.3) as l1l2 x 2 + (l1m2 + l2 m1 ) xy + m1m2 y 2 + (l1n2 + l2 n1 ) x + ( m1n2 + m2 n1 ) y + n1n2 = 0  (3.4) (i.e.) ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 (3.4) where l1l2 = a, m1m2 = b, n1n2 = c

l1m2 + l2 m1 = 2h, l1n2 + l2 n1 = 2 g , m1n2 + m2 n1 = 2 f

The pair of straight lines given by (3.1) and (3.2) is in general represented in the form (3.4). However, we cannot say that every equation of this form will represent a pair of straight lines. We will find the condition that is necessary and sufficient for the equation of the form (3.4) to represent a pair of straight lines.

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3-2

Analytical Geometry

Before that we will see that every second degree homogeneous equation in x and y will represent a pair of straight lines.

3.2  Homogeneous equation of second degree in   x and y Every homogeneous equation of second degree in x and y represents a pair of straight lines passing through the origin. Consider the equation ax 2 + 2hxy + by 2 = 0, a ≠ 0 . 2  y  y 2 Dividing by x , we get b   + 2h   + a = 0. This is a quadratic    x x y y and hence there are two values for , say m1 and m2. Then equation in x x 2  y  y y y  b   + 2h   + a = b  − m1   − m2  .  x  x x x 

(i.e.)  b( y − m1 x )( y − m2 x ) = 0.

But y - m1x = 0 and y - m2x = 0 are straight lines passing through the origin. Therefore, ax2 + 2hxy + by2 = 0 represents a pair of straight lines passing through the origin.

Note 3.2.1: ax 2 + 2hxy + by 2 = b( y − m1 x ) ( y − m2 x ) Equating the coefficients of x2 and xy, we get −2h b a m1 m2 = b

m1 + m2 =

3.3  Angle between the lines represented by   ax 2 + 2hxy + by 2 = 0 Let y - m1x = 0 and y - m2x = 0 be the two lines represented by ax2 + 2hxy + by2 = 0. Then and

M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 2

m1 + m2 = m1 m2 =

−2h b

(3.5)

a b

(3.6)

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Pair of Straight Lines

3-3

Let q be the angle between the lines given by ax2 + 2hxy + by2 = 0. Then the angle between the lines is given by tan q =

± ( m1 + m2 )2 − 4 m1m2 m1 − m2 = 1 + m1m2 m1 + m2 2





a  −2h  −4 ±2   b  b ± 2 h2 − ab = = a a+b 1+ b  ± 2 h2 − ab  ∴q = tan −1   a+b  



(3.7)

The positive sign gives the acute angle between the lines and the negative sign gives the obtuse angle between them.

Note 3.3.1: If the lines are parallel or coincident, then q = 0. Then tanq  = 0. Therefore, from (3.7), we get h2 = ab.

p and so we get from 2 p ± 2 h2 − ab (3.7) tan = . This means a + b = 0. Hence, the condition for 2 a+b the lines to be parallel or coincident is h2 = ab and the condition for the lines to be perpendicular is a + b = 0 (i.e.) Coefficient of x2 + Coefficient of y2 = 0.

Note 3.1.3: If the lines are perpendicular then q =

3.4  Equation for the bisector of the angles between the lines given by ax 2 + 2hxy + by 2 = 0 We will now derive the equation for the bisector of the angles between the lines given by ax2 + 2hxy + by2 = 0. The combined equation of the bisectors x 2 − y 2 xy = . of the angles between the lines given by ax2 + 2hxy + by2 = 0 is a−b h y

B P

Q

A O

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q1 q2

q x

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3-4

Analytical Geometry

Let OA and OB be the two lines y - m1x = 0 and y - m2x = 0 represented by ax2 + 2hxy + by2 = 0. Let the lines OA and OB make angles q1 and q2 with the x-axis. Then, we know that −2h tan q1 + tan q 2 = m1 + m2 = b a tan q1 ⋅ tan q 2 = m1 m2 = b Let q  be the angle made by the internal bisector OP with OX. p Then q + is the angle made by the external bisector OQ with OX. The 2 combined equation of the bisectors is  p  ( y − x tan q )  y − x tan  + q   = 0 2   ⇒

( y − x tan q )( y + x cot q ) = 0

⇒ y − x + xy(cot q − tan q ) = 0 2

2

2  1  2 xy(1 − tan q ) = 2 xy cot q ⇒ x 2 − y 2 = xy  − tan q  =  tan q  2 tan q



But q =

(3.8)

q1 + q 2 or 2q = q1 + q 2 2

−2h b = 2h (3.9) a a−b 1− b x 2 − y 2 xy  a − b = . From (3.8) and (3.9), we get x 2 − y 2 = 2 xy  or   h  a−b h x 2 − y 2 xy = . Hence, the combined equation of the pair of bisectors is a−b h m + m2 tan q1 + tan q 2 tan 2q = tan(q1 + q 2 ) = = 1 = 1 − tan q1 tan q 2 1 − m1m2

Aliter: Let (x1, y1) be a point on the bisector OP. Then

2 y1 y x1 xy 2 tan q tan q = 1 ∴ tan 2q = = = 21 12 2 2 x1 1 − tan q y x1 − y1 1 − 12 x1 Also, 2q  = q1 + q2. According to (3.9) −2h tan q1 + tan q 2 2h tan 2q = = b = a a−b 1 − tan q1 tan q 2 1− b

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(3.10)

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Pair of Straight Lines

3-5

From (3.9) and (3.10),

x1 y1 x12 − y12 x1 y1 2h = = or a−b h x12 − y12 a − b



x 2 − y 2 xy = . a−b h This is the combined equation of the bisectors. The locus of (x1, y1) is

3.5  Condition for general equation of a second degree equation to represent a pair of straight lines We will now derive the condition for the general equation of a second degree equation to represent a pair of straight lines. The condition for the general equation of the second degree ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 to represent a pair of straight lines is abc + 2 fgh − af 2 − bg 2 − ch2 = 0.

Method 1: Consider the general equation of the second degree

ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0

(3.11)

Let lx + my + n = 0 and l1x + m1y + n1 = 0 be the equations of two lines represented by (3.11). Then

ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = (lx + my + n)(l1 x + m1 y + n1 )

Comparing the coefficients, we get ll1 = a lm1 + l1m = 2h mm1 = b ln1 + l1n = 2 g nn1 = c mn1 + m1n = 2 f We know that l l1 0 l1 l 0 m m1 0 × m1 m 0 = 0 n n1 0 n1 n 0

(3.12)

By multiplying the two determinants, we get

2ll1 l1m + lm1 l1n + n1l

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lm1 + l1m 2mm1 m1n + nm1

ln1 + l1n mn1 + nm1 = 0 2nn1

(3.13)

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3-6

Analytical Geometry

Substituting the values from (3.12) in (3.13), we get

2a 2h 2h 2b 2g 2 f

2g 2f = 0 2c

a h g

g f =0 c

(i.e.)

h b f

Expanding the determinant, we get

a(bc − f 2 ) − h( hc − gf ) + g ( hf − bg ) = 0



⇒ abc − af 2 − ch2 + ghf + ghf − bg 2 = 0



⇒ abc + 2 fgh − af 2 − bg 2 − ch2 = 0

This is the required condition.

Method 2: ax2 + 2hxy + by2 + 2gx + 2fy + c = 0



Writing this equation in the form by2 + 2hxy + 2fy + (ax2 + 2gx + c) = 0 and solving for y we get y=



( −2hx − 2 f ) ± 4( f + hx )2 − 4b( ax 2 + 2 gx + c) 2b

−( hx + f ) ± ( h2 − ab) x 2 + 2( hf − bg ) x + ( f 2 − bc) (3.14) b This equation will represent two straight lines if the quadratic expression under the radical sign is a perfect square. The condition for this is 4( hf − bg )2 − 4( h2 − ab)( f 2 − bc) = 0 y=



(i.e.)  h2 f 2 + b 2 g 2 − 2hfbg − ( h2 f 2 − ab 2 c − abf 2 − h2 bc) = 0



(i.e.)  b( abc + 2 fgh − af 2 − bg 2 − ch2 ) = 0

Since b ≠ 0,

abc + 2 fgh − af 2 − bg 2 − ch2 = 0

This is the required condition.

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Pair of Straight Lines

3-7

Method 3:

Let the equation ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 represent a pair of straight lines and let (x1, y1) be their point of intersection. Shifting the origin to the point (x1, y1), we get

a( X + x1 )2 + 2h( X + x1 )(Y + y1 )2 + b(Y + y1 ) + 2 g ( X + x1 ) + 2 f (Y + y1 ) + c = 0



(3.15)

where the new axes OX and OY are parallel to (Ox, Oy).

aX 2 + 2hXY + bY 2 + 2 X ( ax1 + hy1 + g ) + 2Y ( hx1 + by1 + f ) + (3.16) ax12 + 2hx1 y1 + 2 gx1 + 2 fy1 + c = 0

As (3.16) represents a pair of straight lines passing through the new origin, it has to be a homogeneous equation in X and Y. Hence,

ax1 + hy1 + g = 0

(3.17)



hx1 + by1 + f = 0

(3.18)



ax12 + 2hx1 y1 + 2 gx1 + 2 fy1 + c = 0

(3.19)



(i.e.)  ( ax1 + hy1 + g ) x1 + ( hx1 + by1 + f ) y1 + gx1 + fy1 + c = 0 (3.20)

Substituting (3.17) and (3.18) in (3.20), we get gx1 + fy1 + c = 0



(3.21)

Eliminating x1 and y1 from (3.17), (3.18) and (3.21), we get a h g



h b f

g f =0 c

Expanding, we get abc + 2 fgh − af 2 − bg 2 − ch2 = 0.

Note 3.5.1: Solving (3.17) and (3.18), we get

ax1 + hy1 + g = 0



hx1 + by1 + f = 0



⇒

a b 1 = = hf − bg gh − af ab − h2

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3-8

Analytical Geometry

Hence, the point of intersection of the lines represented by (3.11) is  hf − bg gh − af  ,  . ab − h2 ab − h2 

Note 3.5.2: If lx + my + n = 0 and l1x + m1y + n1 = 0 are the two straight lines

represented by (3.11), then lx + my = 0 and l1x + m1y = 0 will represent two straight lines parallel to the lines represented by (3.11) and passing through the origin. Their combined equation is

(lx + my )(l1 x + m1 y ) = 0



(i.e.)  ll1 x 2 + (lm1 + l1m) xy + mn1 y 2 = 0



(i.e.)  ax 2 + 2hxy + by 2 = 0

Therefore, if ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 represents a pair of straight lines, then the equation ax 2 + 2hxy + by 2 = 0 will represent a pair of lines parallel to the lines given by (3.11). We know that every homogeneous equation of second degree in x and y represents a pair of straight lines passing through the origin. We now use this idea to get the combined equation of the pair of lines joining the origin to the point of intersection of the curve ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 and the line lx + my = 1. The equation of the curve

ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 lx + my = 1.

and the line

(3.22) (3.23)

will meet at two points say P and Q. Let (x1, y1) be one of the points of intersection, say P. Then ax12 + 2hx1y1 + by12 + 2gx1 + 2fy1 + c = 0



(3.24)

and lx1 + my1 = 1 Let us homogenise (3.22) with the help of (3.23). Then, we write

ax 2 + 2hxy + by 2 + (2 gx + 2 fy )(lx + my ) + c(lx + my )2 = 0

(3.25)

If we substitute x = x1 and y = y1 in (3.25), we get

ax12 + 2hx1 y1 + by12 + (2 gx1 + 2 fy1 )(lx1 + my1 ) + c(lx1 + my1 )2 = 0

because of (3.23) and (3.24).

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Pair of Straight Lines

3-9

Therefore P(x1, y1) lies on the locus of (3.25). Similarly we can show that the point Q(x2, y2) also lies on the locus of (3.25). However, a second degree homogeneous equation represents a pair of straight lines passing through origin. Hence, (3.25) is the combined equation of the pair of lines OP and OQ. Hence, homogensing the second degree equation (3.22) with the help of (3.23), we get a pair of straight lines passing through the origin.

ILLUSTRATIVE EXAMPLES Example 3.1 The gradient of one of the lines ax 2 + 2hxy + by 2 = 0 is twice that of the other. Show that 8h2 = 9ab.

Solution The equation ax 2 + 2hxy + by 2 = 0 represents a pair of straight lines passing through the origin. Let the lines be y − m1 x = 0 and y − m2 x = 0. Then ax 2 + 2hxy + by 2 = b( y − m1 x )( y = m2 x )



Equating the coefficients of xy and x2 on both sides, we get −2h b a m1m2 = b

m1 + m2 =



Here, it has been given that m2 = 2m1. From (3.26) and (3.27), we get 3m1 = Also 2m12 =

a b

(3.26) (3.27)

−2h −2h or m1 = 3b b

2

a  −2h  ⇒ 2 = ⇒ 8h2 = 9ab  3b  b

Example 3.2 Prove that one of the lines ax 2 + 2hxy + by 2 = 0 will bisect an angle between the coordinate axes if ( a + b)2 = 4 h2 .

Solution Let y − m1 x = 0 and y − m2 x = 0 be the two lines represented by a2 + 2hxy + by 2 = 0.

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3-10

Analytical Geometry

y

y1 − m1x = 0

x

o

−2h a , m1m2 = b b Since one of the lines bisects the angle between the axes, we take m1 = ±1. Then a a m1m2 = ⇒ m2 = ± b b −2h a 2h ⇒ ±1 ± = ⇒ ± ( a + b) = 2h m1 + m2 = b b b Then m1 + m2 =

(i.e.)  ( a + b)2 = 4 h2



Example 3.3 Find the centroid of the triangle formed by the lines given by the equations 12 x 2 − 20 xy + 7 y 2 = 0 and 2 x − 3 y + 4 = 0.

Solution

12 x 2 − 20 xy + 7 y 2 = 0 ⇒ 12 x 2 − 6 xy − 14 xy − 7 y 2 = 0



⇒ 6 x(2 x − y ) − 7 y(2 x − y ) = 0



⇒ (2 x − y )(6 x − 7 y ) = 0

Therefore, the sides of the triangle are represented by

2x − y = 0

(3.28)



6x − 7 y = 0

(3.29)



2x − 3y + 4 = 0

(3.30)

The point of intersection of the lines (3.28) and (3.29) is (0, 0). Let us solve (3.29) and (3.30).

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Pair of Straight Lines

3-11

6x − 7 y = 0 2 x − 3 y = −4



6x − 7 y = 0

(i.e.)  6 x − 9 y = −12 2 y = 12 ⇒ y = 6



\x=7

Thus, the point of intersection of these two lines is (7, 6). Now, let us solve (3.28) and (3.30). 2x − y = 0

2 x − 3 y = −4 2y = 4 ⇒ y = 2



\x=1

Thus, the point of intersection of these two lines is (1, 2). Then, the centroid  0 + 7 + 1 0 + 6 + 2 , of the triangle with vertices (0, 0), (7, 6) and (1, 2) is    3 3   8 8 (i.e.)  ,  .  3 3

Example 3.4

y−

m

2x

=0

Find the product of perpendiculars drawn from the point (x1, y1) on the lines ax 2 + 2hxy + by 2 = 0.

p2

(x1, y1) p1

y − m1x = 0

Solution Let the lines be y − m1 x = 0 and y − m2 x = 0. Then −2h a ; m1m2 = m1 + m2 = b b

M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 11

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3-12

Analytical Geometry

Let p1 and p2 be the perpendicular distances from (x1, y1) on the two lines y − m1 x = 0 and y − m2 x = 0, respectively. p1 =

Then Then

p1 p2 =





y1 − m1 x1 1+ m

2 1

and p2 =

( y1 − m1 x1 )( y1 − m2 x1 )

=

y1 − m2 x1 1 + m22

y12 − ( m1 + m2 ) xy + m1m2 x12

1 + m12 1 + m22 1 + (m m12 + m22 ) + ( m1m2 )2 2h 2h a a y12 + x1 y1 + x12 x1 y1 + x12 y12 + b b b b = = 2 2 2 1 + ( m1 + m2 ) − 2m1m2 + ( m1m2 ) 4 h 2a a 2 1+ 2 − + b b2 b ax12 + 2hx1 y1 + by12 by12 + 2hx1 y1 + ax12 = = b 2 + 4 h2 − 2ab + a 2 ( a − b) 2 + 4 h2

Example 3.5 If the lines ax 2 + 2hxy + by 2 = 0 be the two sides of a parallelogram and the line lx + my = 1 be one of the diagonals, show that the equation of the other diagonal is y(bl − hm) y = ( am − h)lx . Show that the parallelogram is a rhombus if h( a 2 − b 2 ) = ( a − h)lm .

Solution

y−m

2x

=0

C

O

B lx

+m y=

1

y − m1x = 0

A

The diagonal AC not passing through the origin is lx + my = 1. The equation of the lines OA and OC be y − m1 x = 0 and y − m2 x = 0. Then the corresponding coordinates of A are got by solving y − m1 x = 0 and lx + my = 1. m1 1 lx + m( m1 x ) = 1  or  x =  y = l + mm1 l + mm1

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3-13

Pair of Straight Lines

Similarly,

C

is

 1 m1  , \ A is   l + mm1 l + mm1   1 m2   l + mm , l + mm  . The midpoint 2 2

of

AC

is

m1 m2  1 1  + +  l + mm l + mm l + mm l + mm  1 2 1 2 ,     2 2  2l + m( m1 + m2 ) 2mm1m2 + l ( m1 + m2 )  (i.e.)  2(l + mm )(l + mm ) , 2(l + mm )(l + mm )  = say (x1, y1) 1 2 1 2 Since diagonals bisect each other in a parallelogram, the equation of the diy y agonal OB is = 1 x x1  a  −2h  + 2m   l   b  b  y l ( m1 + m2 ) + 2mm1m2 = = 2l + m( m1 + m2 ) x  −2hh  2l + m   b  am − lh =   (i.e.)  y(bl − hm) = x( am − lh) bl − mh If OABC is a rhombus, then the diagonals are at right angles Hence, the product of their slopes is -1.  −l   am − hl  (i.e.)     = −1 ⇒ l ( am − hl ) = m(bl − hm)  m   bl − hm  or

h(l 2 − m2 ) = ( a − b)lm

Example 3.6 Prove that the area of the triangle formed by the lines y = x + c and the straight lines ax 2 + 2hxy + by 2 = 0 is c 2

h2 − ab . a + b + 2h

A

M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 13

0

y

=

−

m

x m2

1x

−

=

y

0

O

y=x+c

B

4/22/2013 9:23:48 PM

3-14

Analytical Geometry

Solution Let the two lines represented by ax 2 + 2hxy + by 2 = 0 be y − m1 x = 0 and y − m2 x = 0. Solving the equations y − m1 x = 0 and y = x + c, we get the coordinates of A to be  c  c mc  mc  , 1  . Similarly B is  A , 2 .  m1 − 1 m1 − 1  m2 − 1 m2 − 1



1 ( x1 y2 − x2 y1 ) 2 mc mc 1 c c = ⋅ 2 − ⋅ 1 2 m1 − 1 m2 − 1 m2 − 1 m1 − 1



=

2 m2 − m1 c2 c 2 ( m2 + m1 ) − 4m1m2 = 2 ( m1 − 1)( m2 − 1) 2 m1m2 − ( m1 + m2 ) + 1



=

c2 2

Area of ∆OAB =

4 h2 − 4 ab h2 − ab = c2 a + b + 2h a + 2h + b

Example 3.7 L and M are the feet of the perpendiculars from (c, 0) on the lines ax2 + 2hxy + by 2 = 0 . Show that the equation of the line LM is ( a − b) x + 2hy + bc = 0.

Solution y M L O

A(c, 0)

x

Let the equation of LM be lx + my = 1. Since L and M are the feet of the perpendiculars from A(c, 0) on the two lines y − m1 x = 0 and y − m2 x = 0, the points O, A, L and M are ­concyclic. The equation of the circle with OA as diameter is x( x − c) + y 2 = 0 or x 2 + y 2 − cx = 0. The combined equation of the lines OL and OM is got by homogenising the equation of the circle with the help of line lx + my = 1.

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Pair of Straight Lines

3-15

Hence, the combined equation of the lines OL and LM is x 2 + y 2 − cx(lx + my ) = 0

(i.e.)  (1 − cl ) x 2 − cmxy + y 2 = 0

But the combined equation of the lines OL and OM is ax 2 + 2hxy + by 2 = 0



Both these equations represent the same lines. Therefore identifying these equations, we get

1 − cl − cm 1 a b−a = = ⇒ 1 − cl = or l = a 2h b b bc −2h 2h −cm = or m = b bc

Therefore, the line lx + my − 1 = 0 is

b−a 2h x− y − 1 = 0. bc bc

(i.e.)  ( a − b) x + 2hy + bc = 0



Example 3.8 Show that for different values of p the centroid of the triangle formed by the straight lines ax 2 + 2hxy + by 2 = 0 are x cos a + y sin a = p lies on the line x( a tan a − h) + y( h tan a − b) = 0.

Solution O

= y

x m2

m

=

1x

y

A

h

D x cosa + y sina = p

B

Let OA and OB be the lines represented by ax 2 + 2hxy + by 2 = 0 and their equations be y − m1 x = 0 and y − m2 x = 0. The equation of the line AB is x cos a + y sin a = p. The coordinates   m1 p p , . The coordinates of B are of A are   cos a + m1 sin a cos a + m1 sin a    m2 p p  cos a + m sin a , cos a + m sin a  . 2 2

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3-16

Analytical Geometry

The midpoint (x1, y1) of AB is m1 p m2 p p p  +, +, D  cos a + m1 sin a cos a + m2 sin a cos a + m1 sin a cos a + m2 sin a ,   2 2 The equation of OD is

y y1 = . x x1

y m1 p(cos a + m2 sin a ) + m2 p(cos a + m1 sin a ) = x p(cos a + m2 sin a ) + p(cos a + m1 sin a ) p cos a ( m1 + m2 ) + 2m1m2 p sin a = = 2 p cos a + p sin a ( m1 + m2 )





 .  

y −2h + 2a tan a a tan a − h = = x 2b + 2h tan a −( h tan a − b)



 −2h   a p cos a  + 2 p sin    b   b  −2h  2 p cos a + p sin a   b 

⇒ x( a tan a − h) + y( h tan a − b) = 0

Example 3.9 Find the condition that one of the lines given by ax 2 + 2hxy + by 2 = 0 may be perpendicular to one of the lines given by a1 x 2 + 2h1 xy + by 2 = 0.

Solution Let y = mx be a line of ax 2 + 2hxy + by 2 = 0. Then

ax 2 + 2hmx 2 + bm2 x 2 = 0 ⇒ a + 2hm + bm2 = 0

(3.31)

1 is a line of a1 x 2 + 2hx1 y1 + b1 y 2 = 0. m mx 2 h Hence, a1 x 2 − 2h 1 + 12 x 2 = 0 or a1m2 − 2h1m + b1 = 0 (3.32) m m m2 m 1 = = From (3.31) and (3.32), we get . 2hb1 + 2h1a aa1 − bb1 −(2bh1 + a1h) Then y = −

Hence, the required condition is ( aa1 − bb1 )2 + 4( ha1 + h1b)(bh1 + a1h) = 0.

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3-17

Pair of Straight Lines

Example 3.10 Two sides of a triangle lie along y 2 − m2 x 2 = 0 and its orthocentre is (c, d). Show that the equation of its third side is (1 − m2 )(cx + dy ) = c 2 − m2 d 2 .

Solution O

0

y 0

E

x m

=

−

x

y

m

=

+

H(c, d )

D ax + by = 1

A

B

Let OA, OB and AB be the lines

y − mx = 0



y + mx = 0



ax + by = 1

Equation of OD is bx − ay = 0. This passes through H(c, d). ∴ bc = ad. Equation of AH is

x − my = c − md , and b =

ad from (1) c

(1)

(3.33)

m   1 The coordinates of A are  ,  a + bm a + bm  That point lies x − my = c − md 1 − m2 = c − md a + bm 1 − m2 a + bm = c − md d (1 − m2 ) c(1 − m2 ) ,b= 2 From (3.33),  a = 2 2 2 c − m2 d 2 c −m d



∴

Hence, the equation of the line AB ( ax + by = 1) becomes (1 - m2)(cx + dy) = c 2 − m2 d 2 .

Example 3.11 Show that the equation m( x 3 − 3 xy 2 ) + y 3 − 3 x 2 y = 0 represents three straight lines equally inclined to one another.

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3-18

Analytical Geometry

Solution y 3 − 3 x 2 y = m(3 xy 2 − x 3 )

Dividing by x3, we get

3

3y  y  −   3 y2  x  x  y  y =m   − 3   = m  2 − 1 or x x 3 y2  x  1− 2 x 3 tan q − tan 3 q y or = m where = tanq x 1 − 3 tan 2 q ⇒ tan3q = m = tan 3a , say 3

∴ 3q = 3a + np where n = 0, 1, 2

q =a +

np 3

∴q = a , a + p /3, a +

2p 3

These values of q  show that the lines are equally inclined to one another.

Example 3.12 Show that the straight lines ( A2 − 3B 2 ) x 2 + 8 ABx + ( B 2 − 3 A2 ) = 0 form with C2 . the line Ax + By + C = 0 an equilateral triangle of area ( 3 A2 + B 2 )

Solution The sides of the triangle are given by

( A + 3B) x + ( B − 3 A) y = 0

(3.34)



( A − 3B) x + ( B + 3 A) y = 0

(3.35)



Ax + by = c

(3.36)

The angle between the lines (3.34) and (3.36) is   A + 3B  A  ±  −   B − 3 A  B  tanq = =± 3  A + 3B  A 1 + ⋅  B − 3A  B

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Pair of Straight Lines

3-19

A (0,0)

q B

f D

C

Similarly the angle between the lines (3.35) and (3.36) is tanf = ± 3 p p and f = Since the three sides form a triangle, q = is the only 3 3 ­possibility. Hence, the triangle is equilateral. 1 Area of triangle = BC ⋅ AD 2 1 = (2 BD ) ⋅ AD 2 = BD ⋅ AD = AD ⋅ AD ⋅ tan 30° AD 2 = 3 C2 = . 3( A2 + B 2 )

Example 3.13 Show that two of the straight lines ax 3 + bx 2 y + cxy 2 + dy 3 = 0 will be perpendicular to each other if a 2 + d 2 + bd + ac = 0.

Solution

ax 3 + bx 2 y + cxy 2 + dy 3 = 0

This being a third degree homogeneous equation, it represents three straight lines passing through origin. Let the three lines be y − m1 x = 0, y − m2 x = 0 and y − m3 x = 0. If m is the slope of any line then dm3 + cm2 + bm + a = 0 −a ∴ m1m2 m3 = d Since the two lines are perpendicular m1m2 = −1



M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 19

(3.37) (3.38) (3.39)

4/22/2013 9:24:07 PM

3-20

Analytical Geometry

From (3.38) and (3.39), we get Since m3 is a root of (3.37)

m3 =

a d

dm33 + cm32 + bm3 + a = 0 3

2

 a  a  a d   + c  + b  + a = 0  d  d  d ⇒ d 2 + bd + ca + a 2 = 0



Exercises 1.

Show that the equation of pair of lines through the origin and perpendicular to the pair of lines ax2 + 2hxy + by2 = 0 is bx2 - 2hxy + ay2 = 0.

2.

Through a point A on the x-axis, a straight line is drawn parallel to the y-axis so as to meet the pair of straight lines ax2 + 2hxy + by2 = 0 in B and C. If AB = BC, prove that 8h2 = 9ab.

3.

From a point A(1, 1), straight lines AL and AM are drawn at right angles to the pair of straight lines 3x2 + 7xy - 2y2 = 0. Find the equation of the pair of lines AL and AM. Also find the area of the quadrilateral ALOM where O is the origin of the coordinate.

4.

Show that the area of the triangle formed by the lines ax2 + 2hxy + by2 = 0 and h2 − ab lx + my = 1 is . 2 am − 2hlm + bl 2

5.

Show that the orthocentre of the triangle formed by the lines ax2 + 2hxy + by2 = 0 x y a+b and lx + my = 1 is given by = = . l m am2 − 2hlm + bl 2

6.

Show that the centroid (x1, y1) of the triangle formed by the lines ax2 + 2hxy + x y 2 by2 = 0 and lx + my = 1 is . = = 2 hl − hm am − hl 3( am − 2hlm + bl 2 )

7.

A triangle has the lines ax2 + 2hxy + by2 = 0 for two of its sides and the point (c, d) for its orthocentre. Prove that the equation of the third side is (a + b) (cx + dy) = ad 2 - 2hbd + bc2.

8.

If the slope of one of the lines given by ax2 + 2hxy + by2 = 0 is k times the other, prove that 4kh2 = abc (1 + k)2.

9.

If the distance of the point (x1, y1) from each of two straight lines through the origin is d, prove that the equation of the straight lines is (x1y - xy1)2 = d 2(x2 + y2).

10.

A straight line of constant length 2l has its extremities one on each of the straight lines ax2 + 2hxy + by2 = 0. Show that the line of midpoint is (ax + by)2 (hx + by) + (ab - h2)2l2a.

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3-21

Pair of Straight Lines

11.

Prove that the triangle formed by the lines ax2 + 2hxy + by2 = 0 and lx + my = 1 is right angled if (a + b)/al2 + 2hlm + bm2) = 0.

12.

Show that if two of the lines ax3 + bx2y + cxy2 + dy3 = 0 make complementary angles with x-axis in anticlockwise direction, then a(a - c) + d(b - d) = 0.

13.

If the slope of the lines given by ax2 + 2hxy + by2 = 0 is the square of the other, show that ab(a + h) - 6ahb + 8h3 = 0.

14.

Show that the line ax + by + c = 0 and the two lines given by (ax + by)2 = c 3(bx - ay)2 form an equilateral triangle of area . 3( a2 + b2 ) If one of the line given by ax2 + 2hxy + by2 = 0 is common with one of the lines of a1x2 + 2h1xy + b1y2 = 0. show that (ab1 - a1b)2 + 4(ah1 - a1h). (bh1 - b1h) = 0.

15.

16.

A point moves so that its distance between the feet of the perpendiculars from it on the lines ax2 + 2hxy + by2 = 0 is a constant 2k. Show that the locus of the point is (x2 + y2)(h2 - ab) = k2[(a - b)2 + 4h2].

17.

Show that the distance from the origin to the orthocentre of the triangle formed 2 2 x y by the lines + = 1 and ax2 + 2hxy + by2 = 0 is ( a + b) pq p + q . 2 2 p q ap − 2hpq + bq

18.

A parallelogram is formed by the lines ax2 + 2hxy + by2 = 0 and the lines through (p, q) parallel to them. Show that the equation of the diagonal not passing through the origin is (2x - p)(ap + hq) + (2y - q)(hp + bq) = 0.

19.

If the lines given by lx + my = 1 and ax2 + 2hxy + by2 = 0 form an isosceles triangle, show that h(l2 - m2) = lm(a - b).

Example 3.14 Find l  so that the equation x2 + 5xy + 4y2 + 3x + 2y + l = 0 represents a pair of lines. Find also their point of intersection and the angle between them.

Solution Consider the second degree terms x2 + 5xy + 4y2. x2 + 5xy + 4y2 = (x + y)(x + 4y) Let the two straight lines be x + y + l = 0 and x + 4y + m = 0. Then x2 + 5xy + 4y2 + 3x + 2y + l  = (x + y + l) (x + 4y + m) Equating the coefficients of x, y and constant terms, we get m + l = 3 m + 4l = 2 lm = l

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(3.40) (3.41) (3.42)

4/22/2013 9:24:08 PM

3-22

Analytical Geometry

−1 1 10 \ m = 3+ = Solving (3.40) and (3.41), we get 3l = -1⇒ l = 3 3 3 10 From (3.42), λ = − 9 10 1 Then the two lines are x + y − = 0, x + 4 y + = 0 (or) 3x + 3y - 1 = 0 3 3 and 3x + 12y + 10 = 0. m − m2 The angle between the lines is given by tanq = 1 1 + m1m2

tan q =

1 − 1/ 4 3  3 = ⇒ q = tan −1    5 1 + 1.1/ 4 5

Example 3.15 Find the value of l  so that the equation lx2 - 10xy + 12y2 + 5x - 16y - 3 = 0 represents a pair of straight lines. Find also their point of intersection.

Solution lx2 - 10xy + 12y2 + 5x - 16y - 3 = 0 Comparing with the equation ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 we get a = l, 2h = -10, b = 12, 2g = 5, 2f = -16, c = -3 5 ⇒ a = l, h = -5, b = 12, g = , f = -8, c = -3 2 The condition for the given equation to represent a pair of straight lines is abc + 2fgh - af 2 - bg2 - ch2 = 0 -36l + 200 - 64l - 75 + 75 = 0 ⇒ l = 2 2 Then 2x - 10xy + 12y2 + 5x - 16y - 3 = (2x - 4y + l) (x - 3y + m) Equating the coefficients of x, y and constant terms, 4 m + 2l = 10 2m + l = 5 or 4 m + 3l = 16 4m + 3l = 16 l=6 lm = −3 −3 −1 = ∴m = 6 2 Therefore, the two lines are x - 2y + 3 = 0 and 2x - 6y - 1 = 0. Solving these  7 two equations, we get the point of intersection as  4,  .  2

Example 3.16 Find the value of l so that the equation x2 - lxy + 2y2 + 3x - 5y + 2 = 0 represents a pair of straight lines.

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Pair of Straight Lines

Solution

3-23

3 −l −5 , g = , h= 2 2 2 2 2 2 abc + 2fgh - af - bg - ch = 0

a = 1, b = 2, c = 2, f =



 −l 2  l2 9  25   5  3  −l   3 ⇒ 4 + 2 −  × ×  − 1 × − 1  − 2   − 2  − 2 × =0   2 2  2   2 4  4 4  4 

⇒ 2l 2 − 15l + 27 = 0



⇒ (2l − 9)( l − 3) = 0

⇒ l = 3,

9 2

Example 3.17 Prove that the general equation of the second degree ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 represents parallel straight lines if h2 = ab and bg2 = af 2. Prove that g 2 − ac . the distance between the two straight lines is 2 a( a + b)

Solution Let the parallel lines be lx + my + n = 0 and lx + my + n1 = 0. Then ax2 + 2hxy + by2 + 2gx + 2fy + c = (lx + my + n) (lx + my + n1) Equating the like terms, we get l2 = a m2 = b nn1 = c

ln1 + nl = 2g mn1 + nm = 2f h2 = ab = l2m2 2

 l ( n + n1 )  bg 2 = m2   2  ∴ bg 2 = af 2 2  m( n + n1 )  2 2 2 l2   = l f = af 2   Also, the distance between the lines lx + my + n = 0 and lx + my + n1 = 0 is n − n1 l 2 + m2

=

( n + n1 )2 − 4 nn1 l 2 + m2 2



=

 2g    − 4c l

M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 23

a+b

=

2 g 2 − ac a( a + b)

4/22/2013 9:24:12 PM

3-24

Analytical Geometry

Example 3.18 If ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 represents two straight lines equidistant from the origin, show that f 4 − g 4 = c(bf 2 − ag 2 ).

Solution Let the two lines represented by the given equation be lx + my + n = 0 and l1x + m1y + n1 = 0. Then

ll1 = a

lm1 + l1m = 2h



mm1 = b

ln1 + l1n = 2g



nn1 = c

mn1 + m1n = 2f

Perpendicular distances from the origin to the two lines are equal. Therefore, n l +m 2

2

=

n1 l + m12 2



n2 (l12 + m12 ) = n12 (l 2 + m2 )



n2 l12 − n12 l 2 = n12 m2 − n2 m12



( nl1 + n1l )( nl1 − n1l ) = ( n1m + nm1 )( n1m − nm1 )

Squaring ( nl1 + n1l )2 [( nl1 + n1l )2 − 4 n1nll1 ] = ( n1m + nm1 )2 ( n1m + nm1 )2 − 4 mm1nn1 

⇒ 4g2(4g2 - 4ac) = 4f 2[4f 2 - 4bc]



⇒ f 4 - g4 = c(bf 2 - ag2)

Example 3.19 If the equation ax2 + 2hxy + by2 + 2gx + by2 + 2gx + 2fy + c = 0 represents two straight lines, prove that the product of the lengths of the perpendiculars from c the origin on the straight lines is . ( a − b) 2 + 4 h2

Solution Let the two lines be lx + my + n = 0 and l1x + my + n = 0. Therefore,

ll1 = a

M03_ANALYTICALGEOMETRY_ISBN_CH03.indd 24

lm1 + l1m = 2h

4/22/2013 9:24:14 PM

Pair of Straight Lines

mm1 = b



3-25

ln1 + l1n = 2g

nn1 = c mn1 + m1n = 2f The product of the perpendiculars from the origin on these lines

n

p. pL =

l +m 2

⋅

n1 l +m 2 1

2 1

nn1

=

l l + m m12 + l 2 m12 + l12 m2 2 2 1

2

nn1

=



2

(ll1 ) + ( mm1 ) + (lm1 + l1m)2 − 2ll1mm 2

2

c

=

a 2 + b 2 − 2ab + 4 h2

=

,

c ( a − b) 2 + 4 h2

Example 3.20 If ax2 + 2hxy + by2 + 2gx + by2 + 2gx + 2fy + c = 0 represents two straight lines, prove that the square of the distance of their point of intersection from the c( a + b ) − f 2 − g 2 origin is . Further, if the two given lines are perpendicular, ab − h2 then prove that the distance of their point of intersection from the origin is f 4 + g4 . b 2 + h2

Solution Let the two straight lines be lx + my + n = 0 and l1x + m1y + n1 = 0.  mn − m1n nl1 − n1l1  , . Their point of intersection is  1  lm1 − l1m lm1 − l1m  Hence, the distance of this point from the origin is given by d2 =

( mn1 − m1n)2 + ( nl1 − n1l )2 (lm1 − l1m)2



=

[( mn1 + m1n)2 − 4 mm1nn1 ] + [(nnl1 + n1l )2 − 4 nn1ll1 ] (lm1 + l1m)2 − 4ll1mm1



=

( 4 f 2 − 4bc) + ( 4 g 2 − 4 ac) f 2 + g 2 − c( a + b) = 4 h2 − 4 ab h2 − ab



=

c( a + b ) − f 2 − g 2 ab − h2

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3-26

Analytical Geometry

If the lines are perpendicular then (a + b) = 0. Then d2 =



0 − ( f 2 + g2 ) f 2 + g2 = − ab − h2 ab − h2

Example 3.21 Show that the lines given by 12x2 + 7xy - 12y2 = 0 and 12x2 + 7xy - 12y2 x + 7y - 1 = 0 are along the sides of a square.

Solution

12x2 + 7xy - 12y2 = 0 12x2 + 7xy - 12y2 - x + 7y - 1 = 0

(3.42) (3.43)

The second degree terms in (3.42) and (3.43) are the same. This implies that the two lines represented by (3.42) are parallel to the two lines represented by (3.43). Hence, these four lines from a parallelogram. Also, in each of the equations coefficient of x2 + coefficient of y2 = 0. Hence, each equation forms a pair of perpendicular lines. Thus, the four lines form a rectangle. The two lines represented by (3.42) are 3x + 4y = 0 and 4x - 3y = 0. The two lines represented by (3.43) are 3x + 4y - 1 = 0 and 4x - 3y + 1 = 0. 1 The perpendicular distance between 2x + 4y = 0 and 3x + 4y - 1 = 0 is . 5 1 The perpendicular distance between 4x - 3y = 0 and 4x - 3y + 1 = 0 is . 5 Hence, the four lines form a square.

Exercises 1.  2.  3. 

Show that the equation 6x2 + 17xy + 12y2 + 22x + 31y + 20 = 0 represents a pair of straight lines and find their equations. Ans.: 2x + 3y + 4 = 0  3x + 4y + 5 = 0 Prove that the equations 8x2 + 8xy + 2y2 + 26x + 13y + 15 = 0 represents two parallel straight lines and find the distance between them. 7 5 Ans.: 2 Prove that the equation 3x2 + 8xy - 7y2 + 21x - 3y + 18 = 0 represents two lines. Find their point of intersection and the angle between them. −3 −5 p Ans.:  ,  ,  2 2 2

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Pair of Straight Lines

4.

5.

6.  7.  8.

3-27

If ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 and ax2 + 2hxy + by2 - 2gx - 2fy + c = 0 each represents a pair of lines, prove that the area of the parallelogram 2c enclosed is . 2 h − ab Show that the equation 3x2 + 10xy + 8y2 + 14x - 22y + 15 = 0 represents two  2 straight lines intersecting at an angle tan −1   .  11 The equation ax2 - 2xy - 2y2 - 5x + 5y + c = 0 represents two straight lines perpendicular to each other. Find a and c. Ans.: a = 2, c = -3 Find the distance between the parallel lines given by 4x2 + 12xy + 9y2 - 6x - 9y + 1 = 0. 3 Ans.: 2 Show that the four lines 2x2 + 3xy - 2y2 = 0 and 2x2 + 3xy - 2y2 -3x + y + 1 = 0 form a square.

9.

Show that the straight lines represented by ax2 + 2hxy + by2 = 0 and those represented by ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 form a rhombus, if (c - h) fg + h(f 2 - g2) = 0.

10.

If the equation ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 represents two straight lines and parallel lines to these two lines are drawn through the origin then show that the area of the parallelogram so formed is

11. 12.

c

. 2 h − ab If the straight lines given by ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 intersects on the y-axis then show that 2fgh - hg2 - ch2 = 0. 2

A parallelogram is such that two of its adjacent sides are along the lines ax2 + 2hxy + by2 = 0 and its centre is (a, b). Find the equation of the other two sides. Ans.: a(x - 2a) + 2h(x - 2a) (y - 2b) + b(y - 2b)2 = 0



Example 3.22 Show that the pair of lines given by ( a − b)( x 2 − y 2 ) + 4 hxy = 0 and the pair of lines given by h( x 2 − y 2 ) = ( a − b) xy are such that each pair bisects the angle between the other pairs.

Solution

( a − b)( x 2 − y 2 ) + 4 hxy = 0

(3.44)



h( x − y ) − ( a − b) xy = 0

(3.45)

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2

2

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3-28

Analytical Geometry

The combined equation of the bisectors of the pair of lines given by (3.44) is x2 − y2 xy = ( a − b) + ( a − b) 2h (i.e.)  h( x 2 − y 2 ) = xy( a − b) which is (3.45). The combined equation of the bisectors of the angle between lines given by x2 − y2 xy = (3.45) is 2h  a − b −  2  (i.e.)  ( x 2 − y 2 )( a − b) = −4 hxy



⇒ ( x 2 − y 2 )( a − b) + 4 hxy = 0



which is (3.44). Hence, each pair bisects the angle between the other.

Example 3.23 If the bisectors of the line x 2 − 2 pxy − y 2 = 0 are x 2 − 2qxy − y 2 = 0 show that pq + 1 = 0.

Solution x2 - 2pxy - y2 = 0 x2 - 2qxy - y2 = 0 The combined equation of the bisectors of (3.46) is

(3.46) (3.47)

x 2 − y 2 xy x2 − y2 xy = ⇒ = a−b h 1+1 −p



⇒ − px 2 + py 2 = 2 xy ⇒ px 2 + 2 xy − py 2 = 0

But equation of the bisector is given by x 2 − 2qxy − y 2 = 0

(3.46) (3.47)

p 1 p = = . 1 −q 1 ∴ pq + 1 = 0

Comparing (3.46) and (3.47), we get

Example 3.24 Prove that if one of the lines given by the equation ax2 + 2hxy + by2 = 0 bisects the angle between the coordinate axes then (a + b)2 = 4h2.

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Pair of Straight Lines

3-29

Solution The bisectors of the coordinate axes are given by y = x and y = -x. If y = x is one of the lines of ax2 + 2hxy + by2 = 0 then ax2 + 2hx2 + bx2 = 0. (i.e.)  a + b = −2h If y = − x is one of the lines of ax2 + 2hxy + by2 = 0, then a + b = 2h. From these two equations, we get ( a + b)2 = 4 h2 .

Example 3.25 Show that the line y = mx bisects the angle between the lines ax2 + 2hxy + by2 = 0 if h(1 - m2) + m(a - b) = 0.

Solution The combined equation of the bisectors of the angles between the lines ax2 2hxy + by2 = 0 is x 2 − y 2 xy = a−b −h If y = mx is one of the bisectors, then it has to satisfy the above equation. x 2 − m2 x 2 mx 2 = a−b −h ⇒ h(1 − m2 ) + m( a − b) = 0



(i.e.) 



Example 3.26 Show that the pair of the lines given by a 2 x 2 + 2h( a + b) xy + b 2 y 2 = 0 is equally inclined to the pair given by ax2 + 2hxy + by2 = 0.

Solution In order to show that the pair of lines given by a 2 x 2 + 2h( a + b) xy + b 2 y 2 = 0 is equally inclined to the pair of lines given by ax 2 + 2hxy + by 2 = 0, we have to show that both the pairs have the same bisectors. The combined equations x2 − y2 xy x 2 − y 2 xy = ⇒ = , of the bisectors of the first pair of lines is 2 a−b h a − b 2 h( a + b) which is the combined equation of the second pair of lines.

Exercises 1.

If the pair of lines x2 - 2axy - y2 = 0 bisects the angles between the lines x2 - 2pxy - y2 = 0 then show that the latter pair also bisects the angle between the former pair.

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3-30

Analytical Geometry

2.

If one of the bisectors of ax2 + 2hxy + by2 = 0 passes through the point of intersection of the lines ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 then show that h( f 2 − g 2 ) + ( a − b) fg = 0.

3.

If the pair of straight lines ax2 + 2hxy + by2 = 0 and bx2 + 2gxy + by2 = 0 be such that each bisects the angle between the other then prove that hg - b = 0.

4.

Prove that the equations 6 x 2 + xy − 12 y 2 − 14 x + 47 y − 40 = 0 and 14 x 2 + xy − 4 y 2 − 30 x + 15 y = 0 represent two pairs of lines such that the lines of the first pair are equally inclined to those of the second pair.

5.

Prove that two of the lines represented by the equation ax4 + bx2y + cx2y2 + dxy 3 + ay 4 = 0 will bisect the angle between the other two if c + ba = 0 and b + d = 0.

Example 3.27 If the straight lines joining the origin to the point of intersection of 3 x 2 − xy + 3 y 2 + 2 x − 3 y + 4 = 0 and 2x + 3y = k are at right angles, prove that 6k 2 − 5k + 52 = 0.

Solution Let

3 x 2 − xy + 3 y 2 + 2 x − 3 y + 4 = 0 2x + 3y = k

(3.48) (3.49)

The combined equation of the lines joining the origin to the point of intersection of the lines given (3.48) and (3.49) is got by homogenising (3.48) with the help of (3.49). Hence, the combined equation of the lines joining the origin to the points of intersection of (3.48) and (3.49) is 2 (2 x + 3 y )  2x + 3y  2 2 3 x − xy + 3 y + (2 x − 3 y ) + 4 =0  k  k (i.e.)

k 2 (3 x 2 − xy + 3 y 2 ) + k ( 4 x 2 − 7 y 2 ) + 4( 4 x 2 + 12 xy + 9 y 2 ) = 0

Since the two straight lines are at right angles, coefficient of x2 + coefficient of y2 = 0

⇒ (3k 2 + 4 k + 16) + (3k 2 − 9k + 36) = 0 ⇒ 6k 2 − 5k + 52 = 0

Example 3.28 Show that the pair of straight lines joining the origin to the point of intersection of the straight lines y = mx + c and the circle x2 + y2 = a2 are at right angles 2c 2 = a 2 (1 + m2 ).

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Pair of Straight Lines

3-31

Solution It is given that x 2 + y 2 = a 2 and y = mx + c. P

y = mx + c Q

O x 2 + y 2 = a2

The combined equation of the lines OP and OQ is given by x2 + y2 = 2 2  y − mx  . a   c 

(i.e.)

c 2 ( x 2 + y 2 ) = a 2 ( y 2 2 xmy + m2 x 2 ) ⇒ x 2 (c 2 − m2 a 2 ) − 2ma 2 xy + y 2 (c 2 − a 2 ) = 0

Since OP and OQ are at right angles, coefficient of x2 + coefficient of y2 = 0 c 2 − m2 a 2 + c 2 − a 2 = 0 ⇒ 2c 2 = a 2 (1 + m2 )



Example 3.29 Show that the join of origin to the intersection of the lines 2x2 - 7xy + 3y2 + 5x + 10y - 25 = 0 and the points at which these lines are cut by the line x + 2y - 5 = 0 are the vertices of a parallelogram.

Solution

2x2 - 7xy + 3y2 + 5x + 10y - 25 = 0 x + 2y = 5 B

O

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(3.50) (3.51) C

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3-32

Analytical Geometry

Let equation (3.50) represents the lines CA and CB and (3.51) represents the line AB. The combined equation of the lines OA and OB is got by homogeniousing (3.50) with the help of (3.51). 2



 x + 2y  x + 2y (i.e.) 2 x 2 − 7 xy + 3 y 2 + (5 x + 10 y )  − 25  =0  5   5 



(i.e.) 2 x 2 − 7 xy + 3 y 2 + ( x + 2 y )2 − ( x + 2 y )2 = 0



⇒ 2 x 2 − 7 xy + 3 y 2 = 0

(3.52)

Since the second degree terms in (3.50) and (3.52) are the same the two lines represented by (3.50) are parallel to the two lines represented by (3.52). Therefore, the four lines form a parallelogram.

Example 3.30 If the chord of the circle x2 + y2 = a2 whose equation is lx + my = 1 subtends an angle of 45° at the origin then show that 4[a 2 (l 2 + m2 ) − 1] = [a 2 (l 2 + m2 ) − 2]2 .

Solution It is given that,

x2 + y2 = a2



lx + my = 1 P R 45

Q

O

The combined equation of the lines OP and OQ is

x 2 + y 2 = a 2 (lx + my )2



⇒ x 2 (1 − a 2 l 2 ) − 2a 2 lmxy + y 2 (1 − a 2 m2 ) = 0

Then tan 45° =

2 a 4 l 2 m2 − (1 − a 2 l 2 )(1 − a 2 m2 ) 2 − a2 (l 2 + m2 )

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3-33

Pair of Straight Lines

[a 2 (l 2 + m2 ) − 2]2 = 4[a 4 l 2 m2 − (1 − a 2 l 2 )(1 − c 2 m2 )]



= 4[a 4 l 2 m2 − 1 + a 2 (l 2 + m2 ) − a 4 l 2 m2 ] = 4[a 2 (l 2 + m2 ) − 1]

Example 3.31 Find the equation to the straight lines joining the origin to the point of interx y section of the straight line + = 1 and the circle 5(x2 + y2 + ax + by) = 9ab a b and find the conditions that the straight lines may be at right angles.

Solution It is given that,

5( x 2 + y 2 ) + 5( ax + by ) = 9ab

(3.53)

x y + = 1 (3.54) a b The combined equation of the lines joining the origin to the points of intersection of (3.53) and (3.54) is





 x y  x y 5( x 2 + y 2 ) + 5( ax + by )  +  = 9ab  +   a b  a b

2

9a  9b     5a 5b  − 18 = 0 x 2 5 + 5 −  + y 2 5 + 5 −  + xy  + b a a   b 

Since the lines are at right angles, coefficient of x2 + coefficient of y2 = 0

9b 9a + 10 − = 0 ⇒ 20 ab − 9b 2 − 9a 2 = 0 a b ⇒ 9( a 2 + a 2 ) − 20 ab = 0 10 −

Example 3.32 The line lx + my = 1 meets the circle x2 + y2 = a2 in P and Q. If O is the origin   1 then show that POQ = 2 cos −1  . 2 2  a l +m 

Solution The perpendicular from the origin to the line lx + my = 1 is OR = OP = a

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1 l 2 + m2

,

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3-34

Analytical Geometry

O a P

q R

Q

lx + my =1

OR 1 = 2 OP a l + m2   1 POQ = 2 cos −1 q = 2 cos −1  2 2  a l +m  POR = q then cosq =



Example 3.33 The straight line y - k = m(x + 2a) intersects the curve y2 = 4a (x + a) in A and C. Show that the bisectors of angle AOC, ‘O’ being the origin, are the same for all values of m.

Solution y2 = 4a(x + a)

Let

y2 = 4ax + 4a2 y - mx = 2am + k



The combined equation of the lines OA and OB is 2



( y − mx )  y − mx  y = 4 ax + 4a2   2am + k  2am + k



(i.e.)  (2am + k )2 y 2 = (2am + k )4 ax( y − mx ) + 4 a 2 ( y − mx )2



2

(i.e.) 

x 2 [4 a 2 m2 − 4 am(2am + k )] + xy[4 a(2am + k ) − 8a2 m] + y 2 [4 a2 − (2am + k )]2 = 0

The combined equation of the bisectors is

x2 − y2 xy = 4 a 2 m2 − 4 am(2am + k ) − 4 a 2 + (2am + k )2 2a(2am + k ) − 4 a 2 m

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3-35

Pair of Straight Lines

x2 − y2 xy = 4 a 2 m2 − 8a 2 m2 − 4 amk − 4 a 2 + 4 a 2 m2 + 4 amk + k 2 2ak

(i.e.)

⇒



x2 y2 xy which is independent of m. = k 2 − 4 a 2 2ak

Example 3.34 Prove that if all chords of ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 subtend a right angle at the origin, then the equation must represent two straight lines at right angles through the origin.

Solution ax2 + 2hxy + by2 + 2gx + 2fy + c = 0



(3.55)

Let the equation of the chord be lx + my = 1



(3.56)

Let the lines (3.55) and (3.56) intersect at P and Q. The combined equation of OP and OQ is ax2 + 2hxy + by2 + (2gx + 2fy) (lx + my) + c(lx + my)2 = 0. Since OPQ = 90°, coefficient of x2 + coefficient of y2 = 0. (a + 2gl + cl2) + (b + 2fm + cm2) = 0



Since l and m are arbitrary, coefficients of l1, l2, m1, m2 and the constant term vanish separately. Since g = 0, f = 0, c = 0 and a + b = 0. Hence, equation (3.55) becomes ax2 + 2hxy + by2 = 0 which is a pair of perpendicular lines through the origin.

Exercises 1.

Show that the line joining the origin to the points common to 3x2 + 5xy + 3y2 + 2x + 3y = 0 and 3x - 2y = 1 are at right angles.

2.

If the straight lines joining the origin to the point of intersection 3x2 - xy + 3y2 + 2x - 3y + 4 = 0 and 2x + 3y = k are at right angles then show that 6k2 - 5k + 52 = 0.

3.

Show that all the chords of the curve 3x2 - y2 - 2x + y = 0 which subtend a right angle at the origin pass through a fixed point.

4.

If the curve x2 + y2 + 2gx + 2fy + c = 0 intercepts on the line lx + my = 1, which subtends a right angle at the origin then show that a(l2 + m2) + 2(gl + fm + 1) = 0.

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3-36

Analytical Geometry

5.

If the straight lines joining the origin to the point of intersection of the line kx + hy = 2hk with the curve (x - h)2 + (y - k)2 = a2 are at right angles at the origin show that h2 + k2 = a2.

6.

Prove that the triangle formed by the lines ax2 + 2hxy + by2 = 0 and lx + my = 1 is isosceles if (l2 - m2)h = (a - b)lm.

7.

Prove that the pair of lines joining the origin to the intersection of the curves x2 y2 + = 1 by the line lx + my + n = 0 are coincident if a2l2 + b2m2 = n2. a2 b2

8.

Show that the straight lines joining the origin to the point of intersection of the curves ax2 + 2hxy + by2 + 2gx = 0 and a1x2 + 2h1xy + b1y2 + 2g1x = 0 will be at right angles if g1(a1 + b1) = g(h1 + b1).

9.

Show that the angle between the lines drawn from the origin to the point of intersection of x2 + 2xy + y2 + 2x + 2y - 5 = 0 and 3x - y + 1 = 0 is  4 6 tan −1  .  13 

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Chapter

4

Circle

4.1  INTRODUCTION Definition 4.1.1:  A circle is the locus of a point in a plane such that its distance from a fixed point in the plane is a constant. The fixed point is called the centre of the circle and the constant distance is called the radius of the circle.

4.2  EQUATION OF A CIRCLE WHOSE CENTRE IS (h, k) AND RADIUS r Let C (h, k) be the centre of the circle and P (x, y) be any point on the circle. CP = r is the radius of the circle. CP 2 = r 2 (i.e.) ( x − h)2 + ( y − k )2 = r 2. This is the equation of the required circle.

C (h, k)

P(x, y)

Note 4.2.1:  If the centre of the circle is at the origin, then the equation of the circle is x 2 + y 2 = r 2 .

4.3  CENTRE AND RADIUS OF A CIRCLE REPRESENTED BY THE EQUATION x 2 + y 2 + 2gx + 2fy + c = 0

x 2 + y 2 + 2 gx + 2 fy + c = 0

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4-2



Analytical Geometry

Adding g 2 + f 2 to both sides, we get x 2 + y 2 + 2 gx + 2 fy + g 2 + f 2 = g 2 + f 2 − c



⇒ ( x + g )2 + ( y + f )2 =

(

g2 + f 2 − c

)

2



(4.1)

This equation is of the form ( x − h)2 + ( y − k )2 = r 2 , which is a circle with centre (h, k) and radius r. Thus, equation (4.1) represents a circle whose centre is (−g, −f  ) and radius

g 2 + f 2 − c.

Note 4.3.1:  A second degree equation in x and y will represent a circle if the coefficients of x2 and y2 are equal and the xy term is absent. Note 4.3.2:

1. If g2 + f  2 - c is positive, then the equation represents a real circle. 2. If g2 + f  2 - c is zero, then the equation represents a point. 3. If g2 + f  2 - c is negative, then the equation represents an imaginary circle.

4.4  LENGTH OF TANGENT FROM POINT P(x1, y1) TO THE CIRCLE x 2 + y 2 + 2gx + 2fy + c = 0 The centre of the circle is C (−g, −f  ) and radius r = the tangent from P to the circle.

g 2 + f 2 − c . Let PT be

T

C

P

Then PT 2 = PC 2 − r 2 = ( x1 + g )2 + ( y1 + f )2 − ( g 2 + f 2 − c) = x12 + 2 gx1 + g 2 + y12 + 2 fy1 + f 2 − g 2 − f 2 + c = x12 + y12 + 2 gx1 + 2 fy1 + c



PT =

Note 4.4.1:

x12 + y12 + 2 gx1 + 2 fy1 + c

(1) If PT 2 > 0 then point P(x1, y1) lies outside the circle.

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4-3

Circle



(2) If PT 2 = 0 then the point P(x1, y1) lies on the circle. (3) If PT 2 < 0 then point P(x1, y1) lies inside the circle.

4.5  EQUATION OF TANGENT AT (x1, y1) TO THE CIRCLE   x 2 + y 2 + 2gx + 2fy + c = 0 The centre of the circle is (−g, −f  ). The slope of the radius CP = P(x

1,

y1 + f . x1 + g

y1 )

T C(−g, −f )

Hence, the equation of tangent at (x1, y1) is (y - y1) = m(x - x1) y − y1 =

−( x1 + g ) ( x − x1 ) ⇒ ( y − y1 )( y1 + f ) = −( x1 + g )( x − x1 ) ( y1 + f )



(i.e.) 



(i.e.  yy1 − y12 + fy − fy1 = − x1 x − gx + x12 + gx1



(i.e.)  xx1 + yy1 + gx + fy = x12 + y12 + gx1 + fy1 Adding gx1 + fy1 + c to both sides,



xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = x12 + y12 + 2 gx1 + 2 fy1 + c = 0

since the point (x1, y1) lies on the circle. Hence, the equation of the tangent at (x1, y1) is xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0. + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0.

4.6  EQUATION OF CIRCLE WITH THE LINE JOINING POINTS A (x1, y1) AND B (x2, y2) AS THE ENDS OF DIAMETER A(x1, y1) and B(x2, y2) are the ends of a diameter. Let P(x, y) be any point on the circumference of the circle. Then APB = 90° (i.e.) AP ⊥ PB.

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4-4

Analytical Geometry

P (x, y)

B (x2, y2)

A (x1, y1)

y − y1 y − y2 = m1; the slope of BP is = m2. x − x1 x − x2 Since AP is perpendicular to PB, m1m2 = −1 The slope of AP is

y − y1 y − y1 = −1 , x − x1 x − x2 (i.e.) ( y − y1 )( y − y2 ) = −( x − x2 )( x − x1 ) (i.e.) ( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) = 0 This is the required equation of the circle. (i.e.)

4.7  CONDITION FOR THE STRAIGHT LINE y = mx + c TO BE A TANGENT TO THE CIRCLE x 2 + y 2 = a 2 Method 1:

The centre of the circle is (0, 0). The radius of the circle is a. If y = mx + c is a tangent to the circle, the perpendicular distance from the centre on the straight line y = mx + c is the radius of the circle. P

C(0, 0)



∴

c 1+ m

2

= ± a. ⇒ c 2 = a 2 (1 + m2 )

This is the required condition.

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Circle

4-5

Method 2: The equation of the circle is

x2 + y2 = a2

(4.2)

y = mx + c

(4.3)

The equation of the line is

The x-coordinates of the point of intersection of circle (4.2) and line (4.3) are given by

x 2 + ( mx + c)2 = a 2 (i.e.) (1 + m2 ) x 2 + 2mcx + (c 2 − a 2 ) = 0

(4.4)

If y = mx + c is a tangent to the circle, then the two values of x given by equation (4.4) are equal. The condition for this is the discriminant of quadratic equation (4.4) is zero. 4 m2 c 2 − 4c(1 + m2 )(c 2 − a 2 ) = 0

(i.e.)  m2 c 2 + c 2 − a2 + m2 c 2 − ma 2 = 0



(4.5)

∴ c = a (1 + m ) 2

2

2

This is the required condition.

Note 4.7.1:  Any tangent to the circle x2 + y2 = a2 is of the form y = mx + a 1 + m2 .

4.8  EQUATION OF THE CHORD OF CONTACT OF TANGENTS FROM (x1, y1) TO THE CIRCLE x 2 + y 2 + 2gx + 2fy + c = 0 Let QR be the chord of contact of tangents from P(x1, y1). Let Q and R be the points (x2, y2) and (x3, y3), respectively. The equations of tangents at Q and R are ) , y2 Q(x 2

P(x1, y1) R(x

3,

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y3 )

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4-6



Analytical Geometry

xx2 + yy2 + g ( x + x2 ) + f ( y + y2 ) + c = 0 xx3 + yy3 + g ( x + x3 ) + f ( y + y3 ) + c = 0

These two tangents pass through the point P(x1, y1). Therefore, x1 x2 + y1 y2 + g ( x1 + x2 ) + f ( y1 x1 x2 + y1 y2 + g ( x1 + x2 ) + f ( y1 + y2 ) + c = 0 and

x1 x3 + y1 y3 + g ( x + x3 ) + f ( y + y3 ) + c = 0

These two equations show that the points (x2, y2) and (x3, y3) lie on the straight line xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0 Hence, the equation of the chord of contact from (x1, y1) is

xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0

4.9  TWO TANGENTS CAN ALWAYS BE DRAWN FROM A GIVEN POINT TO A CIRCLE AND THE LOCUS OF THE POINT OF INTERSECTION OF PERPENDICULAR TANGENTS IS A CIRCLE Let the equation of the circle be x2 + y2 = a2

(4.6)

Let (x1, y1) be a given point. Any tangent to the circle x + y = a2 is y = mx + a 1 + m2 . If this tangent points through (x1, y1), then 2



y1 = mx1 + a 1 + m2 ⇒ ( y1 − mx1 )2 = a 2 (1 + m2 )



m2 ( x 2 − a 2 ) − 2mx1 y1 + y12 − a2 = 0 

2

(4.7)

This is a quadratic equation in m. Hence, there are two values for m, and for each value of m there is a tangent. Thus, there are two tangents from a given point to a circle. Let (x1, y1) be the point of intersection of the two tangents from (x1, y1). If m1 and m2 are the slopes of the two tangents, then 2x y (4.8) m1 + m2 = − 2 1 1 2 x1 − a

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m1m2 =

y12 − b 2  x12 − a 2

(4.9)

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Circle

4-7

If the two tangents are perpendicular, then m1m2 = −1.

∴

y12 − b2 = −1 ⇒ y12 − b 2 = − x12 + a 2 ⇒ x12 + y12 = a 2 + b 2 2 2 x1 − a

The locus of (x1, y1) is x2 + y2 = a2 + b2, which is a circle.

4.10  POLE AND POLAR Definition 4.10.1:  The polar of a point with respect to a circle is defined to be the locus of the point of intersection of tangents at the extremities of a variable chord through that point. The point is called the pole.

4.10.1  Polar of the Point P (x1, y1) with Respect to the Circle   x 2 + y 2 + 2gx + 2fy + c = 0 P(x1, y1) Q

T(h, k)

R

Let the equation of circle be x 2 + y 2 + 2 gx + 2 fy + c = 0 



(4.10)

Let QR be a variable chord through the point P(x1, y1). Let the tangents at Q and R to the circle intersect at T(h, k). Then, QR is the chord of contact of the tangents from T(h, k). Its equation is

xh + yk + g ( x + h) + f ( y + k ) + c = 0

This chord passes through P(x1, y1). Therefore,

x1h + y1k + g ( x1 + h) + f ( y1 + k ) + c = 0 

(4.11)

The locus of (h, k) is

xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0 

(4.12)

Hence, the polar of (x1, y1) is xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0.

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4-8

Analytical Geometry

Note 4.10.1.1: 1. If the point (x1, y1) lies outside the circle, the polar of (x1, y1) is the same as the chord of contact from (x1, y1). If the point lies on the circle, then the tangent at (x1, y1) is the polar of the point P(x1, y1). 2. The point (x1, y1) is called the pole of the line xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0. xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = 0. Line (4.12) is called the polar of the point (x1, y1). 3. The polar of (x1, y1) with respect to the circle x2 + y2 = a2 is xx1 + yy1 = a 2.

4.10.2  Pole of the Line lx + my + n = 0 with Respect to the Circle x2 + y2 = a2 Let (x1, y1) be the pole of the line lx + my + n = 0

(4.13)

with respect to the circle x2 + y2 = a2. Then, the polar of (x, y) is xx1 + yy1 = a 2



(4.14)

Equations (4.13) and (4.14) represent the same line. Therefore, identifying these two equations, we get

x1 y1 − a 2 −la 2 − ma 2 = = ∴ x1 = , y1 = l m n n n

 −la 2 −ma 2  , Hence, the pole of the line lx + my + n = 0 is  . n   n

4.11  CONJUGATE LINES Definition 4.11.1: Two lines are said to be conjugate with respect to the circle x2 + y2 = a2 if the pole of either line lies on the other line.

4.11.1  Condition for the Lines lx + my + n = 0 and l1x + m1y + n1 = 0 to be Conjugate Lines with Respect to the Circle x 2 + y 2 = a2  −la 2 − ma 2  The pole of the line lx + my + n = 0 is  , . Since the two given n   n lines are conjugate to each other, this point lies on the line l1 x + m1 y + n1 = 0.

 −la 2   − ma 2  2 2 ∴ l1  + m1  + n1 = 0 (i.e.) ll1a + mm1a = nn1   n   n 

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4.12  EQUATION OF A CHORD OF CIRCLE   x 2 + y 2 + 2gx + 2fy + c = 0 IN TERMS OF ITS MIDDLE POINT Let PQ be a chord of the circle x 2 + y 2 + 2 gx + 2 fy + c = 0 and R(x1, y1) be its middle point. The equation of any chord through (x1, y1) is

x − x1 y − y1 = = r cos q sin q

(4.15)

P

R(x1, y1)

Q

Any point on this line is x = x1 + r cos q , y = y1 + r sin q . When the chord PQ meets the circle this point lies on the circle. Therefore, ( x1 + r cos q )2 + ( y1 + r sin q )2 + 2 g ( x1 + r cos q ) + 2 f ( y1 + r sin q ) + c = 0 (i.e.) r 2 + 2r[( x1 + g ) cos q + ( y1 + f ) sin q ] + x12 + y12 + 2 gx1 + 2 fy1 + c = 0 The values of r of this equation are the distances RP and RQ, which are equal in magnitude but opposite in sign. The condition for this is the coefficient of r = 0. (4.16) ∴ ( x1 + g ) cos q + ( y1 + f ) sin q = 0  Eliminating cosq and sinq, from (4.15) and (4.16), we get  y − y1   x − x1  =0 + ( y1 + f )  ( x1 + g )   r   r 

(i.e.) xx1 + yy1 + gx + fy = x12 + y12 + 2 gx1 + 2 fy1

Adding gx1 + fy1 + c to both sides, we get

xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c = x12 + y12 + 2 gx1 + 2 fy1 + c

This is the required equation of the chord PQ in terms of its middle point (x1, y1). This equation can be expressed in the form T = S1 where T = xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c and S1 = x12 + y12 + 2 gx1 + 2 fy1 + c.

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4-10

Analytical Geometry

Note 4.12.1:  T is the expression we have in the equations of the tangent  (x1, y1) to the circle S: x 2 + y 2 + 2 gx + 2 fy + c = 0 and S1 is the expression we get by substituting x = x1 and y = y1 in the left-hand side of S = 0.

4.13  COMBINED EQUATION OF A PAIR OF TANGENTS FROM (x1, y1) TO THE CIRCLE x 2 + y 2 + 2gx + 2fy + c = 0 Let the equation of a chord through (x1, y1) be

x − x1 y − y1 = = r cos q sin q

(4.17)

Any point on this line is ( x1 + r cos q , y1 + r sin q ). If this point lies on the circle x 2 + y 2 + 2 gx + 2 fy + c = 0, then ( x1 + r cos q )2 + ( y1 + r sin q )2 + 2 g ( x1 + r cos q ) + 2 f ( y1 + r sin q ) + c = 0

(i.e.) r 2 + 2r[( x1 + g ) cos q + ( y1 + f ) sin q ] + x12 + y12 + 2 gx1 + 2 fy1 + c = 0  (4.18) If chord (4.17) is a tangent to circle (4.18), then the two values of r of this equation are equal. The condition for this is [( x1 + g ) cos q + ( y1 + f ) sin q ]2 = [ x12 + y12 + 2 gx1 + 2 ( y1 + f ) sin q ] = [ x12 + y12 + 2 gx1 + 2 fy1 + c]. 2 2 2 2 (i.e.)  [( x1 + g ) + ( y1 + f ) tan q ] = ( x1 + y + 2 gx1 + 2 fy1 + c) sec q



= ( x12 + y12 + 2 gx1 + 2 fy1 + c) (1 + tan 2 q ) 

But from (4.17) tan q =

(4.19)

y − y1 . x − x1

Substituting this in (4.19), we get 2   y − y 2    y − y1   2 2 1 1 +  = + + + + ( x1 + g ) + ( y1 + f )  ( x y 2 gx 2 fy c )    1 1 1 1 x − x − x x     1  1    

[( x1 + g )( x − x1 ) + ( y1 + f )( y − y1 )]2 = ( x12 + y12 + 2 gx1 + 2 fy1 + c)

[( x − x1 )2 + ( y − y1 )2 ] [( xx1 + yy1 + gx + fy ) − ( x12 + y12 + gx1 + fy1 )]2 =



( x12 + y12 + 2 gx1 + 2 fy1 + c)[ x 2 + y 2 − 2( xx1 + yy1 ) + ( x12 + y12 )]



(i.e.)  (T − S1 )2 = S1 [S − 2T + S1 ] or T 2 = SS1

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4-11

Circle

2 (i.e.) [ xx1 + yy1 + g ( x + x1 ) + f ( y + y1 ) + c]

= ( x 2 + y 2 + 2 gx + 2 gx1 + 2 fy1 + c)( x12 + y12 + 2 gx1 + 2 fy1 + c) This equation is the combined equation of the pair of tangents from (x1, y1).

4.14  PARAMETRIC FORM OF A CIRCLE x = a cosq, y = a sinq satisfy the equation x2 + y2 = a2. This point is denoted by ‘q ’, which is called a parameter for the circle x2 + y2 = a2.

4.14.1  Equation of the Chord Joining the Points ‘p ’ and ‘e ’ on the Circle and the Equation of the Tangent at p The two given points are ( a cos q , a sin q ) and ( a cos f , a sin f ) . The equation of the chord joining these two points is





y − a sin q a(sin q − sin f ) = = x − a cos q a(cos q − cos f )

q +f q −f sin 2 2 q −f q +f −2 sin sin 2 2 2 cos

q +f  q +f  q +f  q +f  x cos  − a cos q cos  = − y sin  + a sin q sin      2   2   2   2  q +f  q +f q +f  q +f   + y sin  = a  cos q cos − sin q sin x cos      2   2   2 2  q +f   = a cos q − .  2 



q +f  q +f  q −f  x cos  + y sin  = a cos     2   2   2  This chord becomes the tangent at ‘q ’ if f = 0. Therefore, the equation of the tangent at ‘q ’ is x cos q + y sin q = a.

ILLUSTRATIVE EXAMPLES Example 4.1 Find the equation of the circle whose centre is (3, −2) and radius 3 units.

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4-12

Analytical Geometry

Solution The equation of the circle is

( x − h)2 + ( y − k )2 = r 2 (i.e.)  ( x − 3)2 + ( y + 2)2 = 32



(i.e.)  x 2 − 6 x + 9 + y 2 + 4 y + 4 = 9 ⇒ x 2 + y 2 − 6 x + 4 y + 4 = 0

Example 4.2 Find the equation of the circle whose centre is (a, −a) and radius ‘a’.

Solution

The centre of the circle is (a, −a). The radius of the circle is a. The equation of the circle is ( x − a)2 + ( y + a)2 = a 2 (i.e.) x 2 − 2ax + a 2 + y 2 + 2ay + a 2 = a 2 (i.e.) x 2 2 2 + a + y + 2ay + a 2 = a 2 (i.e.) x 2 + y 2 − 2ax + 2ay + a 2 = 0.

Example 4.3 Find the centre and radius of the following circles: (i) x 2 + y 2 − 14 x + 6 y + 9 = 0 (ii) 5 x 2 + 5 y 2 + 4 x − 8 y − 16 = 0

Solution (i) x 2 + y 2 − 14 x + 6 y + 9 = 0 Centre is (7, −3) Radius =

g 2 + f 2 = c = 49 + 9 − 9 = 49 = 7 units

(ii) 5 x 2 + 5 y 2 + 4 x − 8 y − 16 = 0 Dividing this by 5, we get x 2 + y 2 +

4 8 16 x− y− =0 5 5 5

−2 4 Centre is  ,   5 5 Radius =

4 16 16 100 + + = = 2 units 25 25 5 25

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Circle

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Example 4.4 Find the equation of the circle whose centre is (2, −2) and which passes through the centre of the circle x 2 + y 2 − 6 x − 8 y − 5 = 0

Solution The centre of the required circle is (2, −2). The centre of the circle x 2 + y 2 − 6 x − 8 y − 5 = 0 is (3, 4). The radius of the required circle is given by r 2 = (2 − 3)2 + ( −2 − 4)2 = 1 + 36 = 37.

(3, 4)

(2, −2)

Therefore, the equation of the required circle is ( x − 2)2 + ( y + 2)2 = 37 (i.e.) x 2 + y 2 − 4 x + 4 y − 29 = 0

Example 4.5 Show that the line 4x - y = 17 is a diameter of the circle x 2 + y 2 − 8 x + 2 y = 0.

Solution The centre of the circle x 2 + y 2 − 8 x + 2 y = 0 is (4, −1). Substituting x = 4 and y = −1 in the equation 4x - y = 17, we get 16 + 1 = 17, which is true. Therefore, the line 4x - y = 17 passes through the centre of the given circle. Hence, the given line is a diameter of the circle.

Example 4.6 Prove that the centres of the circles x 2 + y 2 + 4 y + 3 = 0, x 2 + y 2 + 6 x + 8 y − 17 = 0 2 2 0, x + y + 6 x + 8 y − 17 = 0 and x + y − 30 x − 16 y − 42 = 0 are collinear. 2

2

Solution The centres of the three given circles are A(0, −2), B( −3, −4) and C(15, 8). −2 + 4 2 = The slope of AB is 0+3 3

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4-14

Analytical Geometry

−4 − 8 −12 2 = = . −3 − 15 −18 3 Since the slopes AB and BC are equal and B is a common point, the points A, B and C are collinear. The slope of BC is

Example 4.7 Show that the point (8, 9) lies on the circle x 2 + y 2 − 10 x − 12 y + 43 = 0 and find the other end of the diameter through (8, 9).

Solution Substituting x = 8 and y = 9 in x 2 + y 2 − 10 x − 12 y + 43 = 0, we get 64 + 81 − 80 − 108 + 43 = 0 64 + 81 − 80 − 108 + 43 = 0 (i.e.) 188 − 188 = 0, which is true. Therefore, the point (8,  9) lies on the given circle. The centre of this circle is (5, 6). Let (x,  y) be the other end of the diameter. Then

8+ x 9+ y = 5, = 6 ⇒ 8 + x = 10, 9 + y = 12 2 2 ⇒ x = 2, y = 3

(8, 9)

(5, 6)

(x, y)

Hence, the other end of the diameter is (2, 3).

Example 4.8 Find the equation of the circle passing through the points (1, 1), (2, −1) and (3, 2).

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0 . The circle passes through the points (1, 1), (2, −1) and (3, 2).

∴1 + 1 + 2 g + 2 f + c = 0 4 +1+ 4g − 2 f + c = 0  9 + 4 + 6g + 4 f + c = 0

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Circle

4-15



(i.e.)  2 g + 2 f + c = −2

(4.20)



4 g − 2 f + c = −5

(4.21)



6 g + 4 f + c = −13

(4.22)





(4.23) (4.24)

( 4.20) − ( 4.21) gives − 2 g + 4 f = 3 ( 4.21) − ( 4.22) gives − 2 g − 6 f = 8 ( 4.23) − ( 4.24) gives 10 f = −5 −1 ∴f = 2 From equation (4.20), −2g − 2 = 3; − g =

5 2

From equation (4.20), −5 − 1 + c = −2 ⇒ c = 4 Therefore, the equation of the circle is x 2 + y 2 − 5 x − y + 4 = 0.

Example 4.9 Show that the points (3, 4), (0, 5) (−3, −4) and (−5, 0) are concyclic and find the radius of the circle.

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0. This passes through the points (3, 4), (0, 5) and (−3, -4). Therefore, 9 + 16 + 6 g + 8 f + c = 0, 0 + 25 + 0 + 10 f + c = 0, 9 + 16 − 6 g − 8 f + c = 0   

(i.e.)

6 g + 8 f + c = −25  10 f + c = −25 −6 g − 8 f + c = −25

( 4.25) − ( 4.26) gives 6 g − 2 f = 0 (ie) f = 3 g  ( 4.26) − ( 4.27) gives 12 g + 16 f = 0 (ie) 4 f = −3 g

(4.25) (4.26) (4.27) (4.28) (4.29)

∴12 g = −3 g ⇒ 15 g = 0 ⇒ g = 0 From equation (4.28), f = 0 From equation (4.25), c = 25 Hence, the equation of the circle is x 2 + y 2 − 25 = 0 

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(4.30)

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4-16

Analytical Geometry

Substituting x = −5 and y = 0 in equation (4.30), we get 0 + 25 − 25 = 0, which is true. Therefore, (−5, 0) also lies on the circle. Hence, the four ­given points are concyclic. The centre of the circle is (0, 0) and the radius is  5 units.

Example 4.10 Find the equation of the circle whose centre lies on the line x = 2y and which passes through the points (−1, 2) and (3, −2).

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0. This passes through the points (−1, 2) and (3, −2). Therefore,

1 + 4 − 2g + 4 f + c = 0 9 + 4 + 6g − 4 f + c = 0 −2 g + 4 f + c = −5  6 g − 4 f + c = −13

(i.e.)  Subtracting, we get

-8g + 8f = 8 or −g + f = 1



(−g, −f  ) lies on x = 2y



\ −g = −2f or g = 2f

(4.31)  (4.32) (4.33)

(4.34)

Substituting this in equation (4.33), we get

−2f + f = 1 ⇒ f = −1  \ g = −2

From (4.31), 4 − 4 + c = −5 ⇒ c = ⇒ −5 Hence, the equation of the circle is x 2 + y 2 − 4 x − 2 y − 2 y − 5 = 0.

Example 4.11 Find the equation of the circle cutting off intercepts 4 and 6 on the coordinate axes and passing through the origin.

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0. This passes through the points (0, 0), (4, 0) and (0, 6).

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Circle

4-17

y (0, 6) 6 O



(4, 0)

x

c=0  16 + 8 g + c = 0 36 + 12 f + c = 0 ∴ g = −2, f = −3

 



4

(4.35) (4.36) (4.37)

( 4.35) − ( 4.36) gives 16 + 8 g = 0 ( 4.36) − ( 4.37) gives 36 + 2 f = 0

Thus, the equation of the circle is x 2 + y 2 − 4 x − 6 y = 0.

Example 4.12 Find the equation of the circle concentric with x 2 + y 2 − 8 x − 4 y − 10 = 0 and passing through the point (2, 3).

Solution Two circles are said to be concentric if they have the same centre. Therefore, the equation of the concentric circle is x 2 + y 2 − 8 x − 4 y + k = 0 . This circle passes through (2, 3). ∴ 4 + 9 − 16 − 12 + k = 0 ∴ k = 15 Hence, the equation of the concentric circle is x 2 + y 2 − 8 x − 4 y + 15 = 0 .

Example 4.13 Find the equation of the circle on the joining the points (4, 7) and (−2, 5) as the extremities of a diameter.

Solution The equation of the required circle is ( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) = 0 (i.e.)  ( x − 4)( x + 2) + ( y − 7)( y − 5) = 0

(i.e.)  x 2 − 2 x − 8 + y 2 − 12 y + 35 = 0



(i.e.)  x 2 + y 2 − 2 x − 2 y + 27 = 0.

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4-18

Analytical Geometry

Example 4.14 The equation of two diameters of a circle are 2x + y − 3 = 0 and x − 3y + 2 = 0. If the circle passes through the point (−2, 5), find its equation.

Solution The centre of the circle is the point of intersection of the diameter. 2x + y = 3 x − 3 y = −2 ( 4.38) × 3 gives 6x + 3y = 9 ( 4.39) × 1 gives x − 3 y = −2 Adding these two equations, we get 7x = 7.  ∴ x = 1 From (4.38), y = 1. Hence, the centre of the circle is (1, 1) r = (1 + 2)2 + (1 − 5)2 = 9 + 16 = 5.

( 4.38) ( 4.39)



and

radius

is

Therefore, the equation of the circle is

( x − 1)2 + ( y − 1)2 = 25 x 2 + y 2 − 2 x − 2 y − 23 = 0

Example 4.15 Find the length of the tangent from the point (2, 3) to the circle x 2 + y 2 + 8 x + 4 y + 8 = 0.

Solution The length of the tangent from P(x1, y1) to the circle x 2 + y 2 + 2 gx + 2 fy + c = 0 is given by PT 2 = x12 + y12 + 2 gx1 + 2 fy1 + c. Here, the length of the tangent from P(2, 3) to the given circle is PT = 4 + 9 + 16 + 12 + 8 = 49 = 7 units.

Example 4.16 Determine whether the following points lie outside, on or inside the circle x 2 + y 2 − 4 x + 4 y − 8 = 0 : A(0,1), B(5, 9), C ( −2, 3).

Solution The equation of the circle is x 2 + y 2 − 4 x + 4 y − 8 = 0.

AT12 = 0 + 1 − 0 + 4 − 8 = −3 < 0

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Circle

4-19

BT22 = 25 + 49 − 20 + 28 − 8 = 64 > 0 CT32 = 4 + 9 + 8 + 12 − 8 = 25 > 0



Therefore, point A lies inside the circle. Points B and C lie outside the circle.

Example 4.17 Find the equation of the tangent at the point (2, −5) on the circle x 2 + y 2 − 5 x + y − 14 = 0.

Solution Given x 2 + y 2 − 5 x + y − 14 = 0 (2, −5)

(5/2, −1/2)

5 −1 Centre is  ,  . 2 2  1 2 =9 Slope of the radius = 5 2− 2 −1 Slope of the tangent = 9 Therefore, the equation of the tangent is −5 +



−1 ( x − 2) 9 ⇒ 9 y + 45 = − x + 2 or x + 9 y + 43 = 0 y − y1 = m( x − x1 ) (i.e.) y + 5 =

Example 4.18 Find the length of the chord of the circle x 2 + y 2 − 4 x − 6 y − 3 = 0 given that (1, 1) is the midpoint of a chord of the circle.

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4-20

Analytical Geometry

Solution Centre of the circle is (2, 3) and radius r = 4 + 9 + 3 = 4. Point M (1, 1) is the midpoint of the chord AB. M (1, 1)

A

B

4 C (2, 3)

CM 2 = (2 − 1)2 + (3 − 1)2 = 1 + 4 = 5 ∴ AM 2 = r 2 − CM 2 = 16 − 5 = 11



∴ AM = 11 Therefore, the length of the chord = 2 11 units.

Example 4.19 Show that the circles x 2 + y 2 − 2 x + 6 y + 6 = 0 and x 2 + y 2 − 5 x + 6 y + 15 = 0 touch each other internally.

Solution For the circle x 2 + y 2 − 2 x + 6 y + 6 = 0, centre is A(1, −3) and radius r1 = 1 + 9 − 6 = 2 units For the circle x 2 + y 2 − 5 x + 6 y + 15 = 0, 5 centre is B  , −3 and radius r2 = 2  units

25 + 9 − 15 = 4

25 + 36 − 60 1 = 4 2

2

3  5 Distance between the centres is AB = 1 −  + ( −3 + 3)2 =  2 2

∴ r1 − r2 = 2 −

1 3 = 2 2

Thus, the distance between the centres is equal to the difference in radii. Hence, the two circles touch each other internally.

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Circle

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Example 4.20 The abscissa of the two points A and B are the roots of the equation x 2 + 2 x − a 2 = 0 and the ordinates are the roots of the equation  y 2 + 4 y − b 2 = 0. Find the equation of the circle with AB as its diameter. Also find the coordinates of the centre and the length of the radius of the circle.

Solution Let the roots of the equation x 2 + 2 x − a 2 = 0 be a and b. a + b = −2 

Then

ab = −a  2



(4.40) (4.41)

Let g, d  be the roots of the equation x 2 + 4 y − b 2 = 0. Then g  + d  = −4 gd  = −b2 The coordinates of A and B are (a , g  ) and ( b , d  ). The equation of the circle on the line joining the points A and B as the ends of a diameter is ( x − a )( x − b ) + ( y − g )( y − d ) = 0.

⇒ x 2 − (a + b ) x + ab + y 2 − (g + d ) y + g d = 0 ⇒ x 2 + y 2 + 2 x + 4 y − a2 − b2 = 0

The centre of the circle in (−1, −2) and the radius = 1+ 4 + a 2 + b 2 = a 2 + b 2 + 5 .

Example 4.21 Find the equation of a circle that passes through the point (2, 0) and whose centre is the limit point of the intersection of the lines 3x + 5y = 1 and (2 + c) x + 5c 2 y = 1 as c → 1.

Solution The centre of the circle is the point of intersection of the lines 3x + 5 y = 1  (2 + c) x + 5c y = 1  as  c → 1 2



(4.42) (4.43)

c2 − 1 3c 2 − (2 + c) 2 (c − 1) (c + 1) c −1 c +1 . x= 2 = = 3c − c − 2 (c − 1) (3c − 2) 3c + 2

(4.42) × c 2− (4.43) gives [3c 2 − (2 + c)]x = c 2 − 1 ⇒ x =

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4-22

Analytical Geometry

As c → 1, the x-coordinate of the centre is From (4.42), y =

1  6  −1 1− = 5  5  25

2 . 5

2 −1 Hence, the centre of the circle is  ,  .  5 25 

2 −1 Radius is the length of the line joining the points (2, 0) and  ,  .  5 25  2

2

2  1 1601  r = 2 −  + 0 +  =  5  25  625



2

2

2  1 1601  Therefore, the equation of the circles is  x −  +  y +  =  5  25  625 x2 + y2 −



4x 2 4 1 1601 − − =0 y+ + 5 25 25 625 625

(i.e.) 25 x 2 + 25 y 2 − 20 x + 2 y − 60 = 0



Example 4.22 Find the length intercepted on the y-axis by the chord of the circle joining the points (−4, 3) and (12, −1) as diameter.

Solution The equation of the circle is ( x + 4)( x − 12) + ( y − 3)( y + 1) = 0 (i.e.)

x 2 − 8 x − 48 + y 2 − 2 y − 3 = 0 x 2 + y 2 − 8 x − 2 y − 51 = 0

If y1 and y2 are the y-coordinates of the point of intersection of the circle and y-axis, then y1 + y2 = 2 y1 y2 = −51

∴( y1 − y2 )2 = ( y1 + y2 )2 − 4 y1 y2 = 4 + 204 = 208 y1 − y2 = 208 = 4 13 units

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Example 4.23 The rods whose lengths are a and b slide along the coordinate axes in such a way that their extremities are concyclic. Find the locus of the centre of the circle.

Solution Let AB and CB be the portion of x-axis and y-axis, respectively, intercepted by the circle. Let P ( x1 , y1 ) be the centre of the circle. Draw PL and PM perpendicular to x-axis and y-axis, respectively. Then, by second property y D

M C O

P (x1, y1) L

A

B







x

a b OA.OB = OC .OD = r 2 OA = x1 − , OC = y1 − 2 2 a b OB = x1 + , OD = y1 + 2 2 a  a  b  b  ∴  x1 −   x1 +  =  y1 −   y1 +   2  2  2  2

( a2 − b2 ) =0 4 The locus of P ( x1 , y1 ) is 4( x 2 − y 2 ) = a 2 − b 2 .

∴ x12 − y12 −

Example 4.24 Show that the circles x 2 + y 2 − 2 x − 4 y = 0 and x 2 + y 2 − 8 y − 4 = 0 touch each other. Find the coordinates of the point of contact and the equation of the common tangents.

Solution

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 23

x 2 + y 2 − 2 x − 4 y = 0 x2 + y 2 − 8 y − 4 = 0

(4.44) (4.45)

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4-24

Analytical Geometry

The centres of these two circles are C1 (1, 2) and C2 (0, 4). The radii of the two circles are

r1 = 1 + 4 = 5 r2 = 0 + 16 + 4 = 20 = 2 5

The distance between the centres is

C1C2 = (1 − 0)2 + (2 − 4)2 = 5

∴ r1 − r2 = C1C2 . Hence, the circles touch each other internally. The point of contact C divides C1C2 internally in the ratio 1 : 1. (0, 4)

5

(1, 2)

C2

C1

5 C

If C is the point ( x1 , y1 ) then

x1 + 0 y +4 = 1, 1 =2 2 2

(i.e.) x1 = 2, y1 = 0

2−4 = −2. 1− 0 Hence, the slope of the common tangent is 1/2. The equation of the common 1 tangent is y − 0 = ( x − 2) or x − 2 y − 2 = 0 2

∴C(2, 0) is the point of contact. The slope of C1C2 =

Example 4.25 Show that the general equation through the point A( x1 , y1 ) and x ( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) + k x1 x2

of the circle that ­passes B( x2 , y2 ) may be written as y 1 y1 1 = 0. y2 1

Solution Let A( x1, y1 ) and B( x2 , y2 ) be the two points on the circumference of the circle and A( x1, y1 ) be any point on the circumference.

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Circle

4-25

P(x, y)

A(x1, y1)

B(x2, y2)

Let APB = q . The slope of AP and BP are m1 = m2 =

( m − m2 ) y − y2 ⇒ tan q = ± 1 1 + m1m2 x − x2

y − y2 (m − y − y1 ⇒ tan q = ± 1 and m2 = x − x2 1+ m x − x1

y − y1 y − y2 − x − x1 x − x2 ⇒ tanq = ± y − y1 y − y2 1+ × x − x1 x − x2



or ( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) ± cot ∞[( x − x2 )( y − y1 ) − ( x − x1 )( y − y2 )] (i.e.)

( x − x1 )( y − y1 ) + ( x − x2 )( y − y2 ) ± cot ∞[ xy − xy2 − x2 y + x2 y2 − xy + xy1 + x2 y − x2 y1 ] = 0

(i.e.)

( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) ± cot ∞[ x( y1 − y2 ) − y( x1 − x2 ) + x1 y2 − x2 y1 ] = 0

(i.e.)

x ( x − x1 )( x − x2 ) + ( y − y1 )( y − y2 ) + k x1 x2

y 1 y1 1 = 0 y2 1

Example 4.26 Show that if the circle x 2 + y 2 = a 2 cuts off a chord of length 2b on the line y = mx + c, then c 2 = (1 + m)2 ( a 2 − b 2 ) .

Solution Given x 2 + y 2 = a 2 . The centre of the circle is (0, 0). Radius = r = a. Draw OL perpendicular to AB. Then, L is the midpoint of AB.

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4-26

Analytical Geometry

O(0, 0) a A



b L

AL = b , OL =

B y = mx − c

c 1 + m2

, OA = a

Then AL2 = OA2 − OL2



b2 = a2 −

c2 1 + m2

∴ c 2 = (1 + m2 )( a 2 − b 2 ).

Example 4.27 A point moves such that the sum of the squares of the distances from the sides of a square of side unity is equal to a. Show that the locus is a circle whose centre coincides with the centre of the square.

Solution Let the centre of the square be the origin. Let P(x, y) be any point. Then, the 1 1 1 1 equation of the sides are x − = 0, x + = 0, y − = 0 and y + = 0. 2 2 2 2 y D

C P (x, y) x

O A

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B

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Circle

4-27

Sum of the perpendicular distances from P on the sides is equal to a 2 2 2 2 1  1  1  1  ∴ x −  +  x +  +  y −  +  y +  = 9  2  2  2  2 x2 + y2 − 1 = 0 Hence, the locus of P is the circle x 2 + y 2 − 1 = 0 . The centre of the circle is (0, 0), which is the centre of the square.

Example 4.28 If the lines l1 x + m1 y + n1 = 0 and l2 x + m2 y + n2 = 0 cut the coordinate axes at concyclic points, prove that l1l2 = m1m2.

Solution

−n1 −n and 1 . l1 m1 − n1 −n1 . If the and OM1 = If the line meets the axes at L1 and M1, then OL1 = l1 m1 second line meets the axes at L2 and M2, then Given l1 x + m1 y + n1 = 0. The intercepts of the line on the axis are

M2

L1

O

L2

M1

OL2 =

− n2 −n and OM 2 = 2 . Then OL1 .OL2 = OM1 .OM 2 . l2 m2

 −n   −n   −n   −n  (i.e.)   1   2  −  1   2  = 0  l1   l2   m1   m2  (i.e.)  l1l2 = m1m2

Example 4.29 Show that the locus of a point whose ratio of distances from two given points is constant is a circle. Hence, show that the circle cannot pass through the given points.

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4-28

Analytical Geometry

Solution Let the two points A and B be chosen in the x-axis and the midpoint of AB be (0, 0). Then let A(a, 0) and B(−a, 0). Given that PA = K ⋅ PB ⇒ PA2 = K 2 PB 2 where k is a constant. ( x − a)2 + ( y − 0)2 = K 2 [( x + a)2 + y 2 ] In this equation, the coefficients of x2 and y2 are the same and there is no xy term. Therefore, the locus of P is a circle. If A(a, 0) lies on this circle, then O = K 2 [4 a 2 ] ⇒ a = 0 or k = 0, which are not possible. Therefore, the point A does not lie on the circle. Similarly, the point B (−a, 0) also does not lie on the circle.

Example 4.30 Find the equation of the circle whose radius is 5 and which touches the circle x 2 + y 2 − 2 x − 4 y − 20 = 0 at the point (5, 5).

Solution Given x 2 + y 2 − 2 x − 4 y − 20 = 0. Centre is (1, 2) and radius = r = 1 + 4 + 20 = 5 Let the centre of the required circle be (x1, y1). The point of contact is the midpoint of AB. (i.e.) 

( x + 1) ( y + 2) = 5, =5 2 2 (1, 2) 5 A

5 C

B

\ x = 9 and y = 8 Thus, B is (9, 8). Hence, the equation of the required circle is ( x − 9)2 + ( y − 8)2 = 25 2 2 (i.e.) x + y − 18 x − 16 y + 120 = 0

Example 4.31 One of the diameters of the circle circumscribing the rectangle ABCD is 4 y = x + 7. If A and B are the points (−3, 4) and (5, 4), respectively, find the area of the rectangle ABCD.

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Circle

4-29

Solution Let P (x1, y1) be the centre of the circle and 4 y = x + 7 be the equation of the diameter of BD.

D

C P(x1,y1)

A (−3, 4)

B(5, 4)

L (1, 0)

The midpoint of AC is (1, 1). The slope of AB is 0. Therefore, the slope of PL is ∞. y −0 Hence 1 =É ∴ x1 = 1 x1 − 1 (x1, y1) lies on 4 y = x + 7

⇒ 4 y1 = 8 or y1 = 2

∴ P is (1, 2). Now AB 2 = ( −3 − 5)2 + ( 4 − 4)2 = 64 or AB = 8

BC = 2 ⋅ PL = 2 (1 − 1)2 + (2 − 0)2 = 2 × 2 = 4

Hence, the area of the rectangle ABCD = 8 × 4 = 32 sq. cm.

Example 4.32 Find the equation of the circle touching the y-axis at (0, 3) and making an intercept of 8 cm on the x-axis.

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0. Centre is  (−g, −f  ), Thus, −f = 3 or f = −3. When the circle meets the x-axis, y = 0. y

C(0, 3) O A

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 29

(−g, −f ) B

x

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4-30

Analytical Geometry

∴ x 2 + 2 gx + c = 0



If x1, x2 are the x coordinates of A and B then x1 + x2 = −2 g x1 x2 = c But OA ⋅ OB = 9 and x2 − x1 = 8 x1 x2 = 9 Also f = -3 ∴ ( x1 − x2 )2 = ( x1 + x2 )2 − 4 x1 x2 64 = 4g2 - 36 ⇒ g = ±5 Hence, the equation of the circle is x 2 + y 2 ± 10 x − 6 y + 9 = 0.

Example 4.33 Find the equation of the circle passing through the point (-4, 3) and touching the lines x + y = 2 and x − y = 2.

Solution x + y = 2 and x − y = 2 intersect at the point (2, 0).

(2, 0)

Moreover, these lines are perpendicular and their slopes are 1 and −1. So, they make 45° and 135° with the x-axis. Hence one of the bisectors is the x-axis and centre lies on one of the bisectors. If x 2 + y 2 + 2 gx + 2 fy + c = 0 is the equation of the circle, then f = 0. Also the perpendicular distance from (−g, 0) to the tangents is equal to the radius. −g − 2 = ± g2 − c ∴ 2 (i.e.) ( g + 2)2 = 2( g 2 − c)

∴ g 2 − 4 g − 4 − 2c = 0

Since (-4, 3) lies on the circle 16 + 9 − 8 g + c = 0 8 g = 25 + c or c = 8 g − 25

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 30

(4.46) (4.47)

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Circle

Hence, equation g 2 − 20 g + 46 = 0.

(4.46)

g 2 − 4 g − 4 − 16 g + 50 = 0

becomes

4-31

or

20 ± 400 − 4 × 46 2



∴g =



=



= 10 ± 3 6

20 ± 216 2

From (4.47), c = 8 g − 25 = 80 + 24 6 − 25

= 55 ± 24 6

Thus, there are two circles whose x 2 + y 2 + 2(10 ± 3 6 ) x + 55 ± 24 6 = 0

equations

are

given

by

Example 4.34 A is the centre of the circle x 2 + y 2 − 2 x − 4 y − 20 = 0 . If the tangents drawn at the points B(1, 7) and D(4, −2) on the circle meet at the point C, then find the area of the quadrilateral ABCD.

Solution

x 2 + y 2 − 2 x − 4 y − 20 = 0

Centre of this circle is (1, 2) r = 1 + 4 + 20 = 5



4, D(

−2)

A (1, 2)

C (16, 7)

B (1

, 7)

The equations of tangents at (1, 7) and (4, -2) to the circle are x + 7 y − ( x + 1) − 2( y + 7) − x + 7 y − ( x + 1) − 2( y + 7) − 20 = 0 (i.e.) 5 y − 35 = 0 ⇒ y = 7 and

4 x − 2 y − ( x + 4) − 2( y − 2) − 20 = 0

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4-32

Analytical Geometry

(i.e.) 3 x − 4 y − 20 = 0



Since y = 7, x = 16. Hence, the point C is (16, 7). Area of the quadrilateral ABCD = 2 × area of ∆ABC

1 = 2 × [1(7 − 7) + 16(7 − 2) + 1(2 − 7)] 2



= 0 + 80 − 5 = 75 sq. units

Example 4.35 From the point A(0, 3) on the circle x 2 + 4 x + ( y − 3)2 = 0 , a chord AB is drawn and extended to a point M such that AM = AB . Find the equation of the locus of M.

Solution

A(0, 3) C

M(x1, y1)

B



AM = 2. AB



x12 4 x1  y1 + 3  + + − 3 = 0  4 2  2



(i.e.) x12 + 8 x1 + ( y1 − 3)2 = 0

 x y + 3 . Hence, B is the midpoint of AM. Then the coordinates of B are  1 , 1 2 2  2 2 x + 4 x + ( y − 3 ) = 0 . This point B lies on the circle 2

Therefore, the locus of (x1, y1) is x 2 + y 2 + 8 x − 6 y + 9 = 0.

Example 4.36 AB is a diameter of a circle, CD is a chord parallel to AB and 2CD = AB. The tangent at B meets the line AC produced at E. Prove that AE = 2. AB.

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Circle

4-33

Solution Let the equation of the circle be x 2 + y 2 = a 2 and PQ be the diameter along the x-axis. CD is parallel to AB. Let AB = 2a and points A and B be (a, 0) and (−a, 0), respectively. Also 1 CD = AB. 2

B

A D

C

E

a Hence, the coordinates of C are  , − h . 2  The point lies on the circle x 2 + y 2 = a 2 .   a2 3 + h2 = a 2 or h = a and the point C  a , 3 a 2 2  4 2 3 0+ a y−0 2 = or y = 3( x − a). The equation of AE is a x−a a− 2 Hence, the point of intersection E is ( − a, −2 3a).

∴



(

AE 2 = ( a + a)2 + 0 + 2 3a ∴ AE = 4 a and AB = 2a ∴ AE = 2. AB

)

2

= 16a 2

Example 4.37 Find the area of the triangle formed by the tangents from the point (h, k) to the circle x 2 + y 2 = a 2 and their chord of contact.

Solution The equation of the circle is x 2 + y 2 = a 2 . Let AB be the chord of contact of tangents from C (x1, y1). Then the equation of AB is xx1 + yy1 = a 2.

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4-34

Analytical Geometry

A a C(x1, y1)

L

O

B

We know that OC is perpendicular to AB. Let AB and OC meet at L. a2



OL =



AL = OA2 − OL2

x2 + y2



= a2 −



=a ∴ AB = 2 AL = 2a



a2 x12 + y12

x12 + y12 − a 2 x12 + y12 x12 + y12 − a 2 x12 + y12

The perpendicular distance from C on AB x 2 + y12 − a 2 CL = 1 x12 + y12 Hence, the area of D ABC =

1 ⋅ AB ⋅ CL 2

x 2 + y 2 − a 2 ( x 2 + y12 − a 2 ) 1 ⋅ 2a 1 2 1 2 ⋅ 1 2 x1 + y1 x12 + y12



=



a( x12 + y12 − a2 ) 2 = x12 + y12

3

Example 4.38 Let a circle be given by 2 x( x − a) + y(2 y − b) = 0 , ( a, b ≠ 0) . Find the condition on a and b if two chords each intersected by the x-axis can be drawn to  b the circle from  a,  .  2

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4-35

Solution

2 x ( x − a) + y ( 2 y − b) = 0

by =0 2 The chord is bisected by the x-axis. Let the midpoint of the chord be (h, 0). The equation of the chord is x 2 + y 2 − ax −



T = S1



a b xh + y(0) − ( x + h) − ( y + 0) = h2 − ah 2 4

 b This chord passes through  a,  .  2

a b h ∴ ah − ( a + h) − ⋅ = h2 − ah 2 4 2 (i.e.) h2 −

3ah a 2 bh + + =0 2 2 8

Since the chord meets the x-axis at two reals, Discriminant > 0  a2 b2  9a 2 ∴ − 4 +  > 0 4 8  2

(i.e.) 9a 2 − 8a 2 − 2b 2 > 0



⇒ a 2 > 2b 2

Example 4.39 Find the condition that the chord of contact from a point to the circle x 2 + y 2 = a 2 subtends a right angle at the centre of the circle.

Solution The equation to the chord of contact from (x1, y1) to the circle is

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 35

x 2 + y 2 = a2 

(4.48)

xx1 + yy1 = a 2

(4.49)

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4-36

Analytical Geometry

Then the combined equation to OA and OB is got by homogenizing equation (4.48) with the help of equation (4.49).

B

A

 xx + yy  The combined equation of OA and OB is x 2 + y 2 = a 2  1 2 1   a 

2

(i.e.)  x 2 [a 2 − x12 ] + y 2 [a 2 − y12 ] − 2 x1 y1 xy = 0 Since AOB = 90° , coefficient of x2 + coefficient of y2 = 0.

∴ a 2 − x12 + b 2 − y12 = 0 x12 + y12 = a 2 + b 2



Example 4.40 If y = mx be the equation of a chord of the circle whose radius is a, the origin being one of the extremities of the chord and the axis being a diameter of the circle, prove that the equation of a circle of which this chord is a diameter is (1 + m2 )( x 2 + y 2 ) − 2a( x + my ) = 0 .

Solution Let a be the radius of the circle. Thus (a, 0) is the centre of the circle. The equation of the circle is

( x − a)2 + y 2 = a 2 ⇒ x 2 + y 2 − 2ax = 0

When y = mx meets the circle x 2 + m2 x 2 − 2ax = 0.

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 36

∴ x = 0 or x =

2a 1 + m2

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4-37

y

=

m x

Circle

a

(0, 0)

(a, 0)

2a 2am  Therefore, the extremities of this chord are (0, 0) and  . Then, ,  1 + m2 1 + m2  the equation of the circle with the chord as a diameter is



2a   xx − +  1 + m2 

2am   y y −  =0  1 + m2 

(i.e.) ( x 2 + y 2 )(1 + m2 ) − 2a( x + my ) = 0

Example 4.41 Find the equation to the circle that passes through the origin and cuts off equal chords of length a from the straight lines y = x and y = − x.

Solution Let the lines y = x and y = − x meet the circle at P, P′ and Q, Q′, respectively. Q

y

P

O

P′

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 37

45° 45°

x

Q′

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4-38

Analytical Geometry

Then  a ,  2  −a ,  2

OP = OQ = a = OPÄ = OQÄ . The coordinates of P and P′ are a   −a −a  ,  and   . Similarly the coordinates of Q and Q′ are 2 2 2 a   a −a  ,  and   . There are four circles possible having centres 2 2 2

 a   −a   a   −a  at  ,0 , , 0 , 0,  ,  0, .  2   2   2  2 Hence, the equations of the four circles are given by

x 2 + y 2 ± 2ax = 0 and x 2 + y 2 ± 2ay = 0

Example 4.42 Find the locus of the midpoint of chords of the circle x 2 + y 2 = a 2 , which subtends a right angle at the point (c, 0).

Solution Since AB subtends 90° at C(c, 0), PA = PB = PC . Let P be the point (x1, y1). y

P

B

O

C (c, 0)

A x

Since P is the midpoint of the chord AB, CP ⊥ AP AP 2 = AC 2 − CP 2 = a 2 − ( x12 + y12 )

PC 2 = ( x1 − c)2 + y12

Since APC = 90° , PC = AP. ∴ ( x1 − c)2 + y12 = a 2 − ( x12 + y12 )

2( x12 + y12 ) − 2cx1 + c 2 − a 2 = 0

The locus of (x1, y1) is 2( x 2 + y 2 ) − 2cx1 + (c 2 − a 2 ) = 0.

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Circle

4-39

Example 4.43 Find the equations of the circles that touch the coordinate axes and the line x = a.

Solution y = 0, x = 0 and x = a are the tangents to the circle. There are two circles as shown in the figure. y x=a

x

a a a The centres are  , ±  and radius . The equations of the circles are 2 2 2 2

2

a a    a  x −  +  y ±  =   2 2 2

2

⇒ x 2 + y 2 − ax ± ay +



a2 =0 4

⇒ 4( x 2 + y 2 ) − 4 ax ± 4 ay + a 2 = 0

Example 4.44

Find the shortest distance from the point (2, -7) to the circle x 2 + y 2 − 14 x − 10 y − 151 = x + y − 14 x − 10 y − 151 = 0. 2

2

Solution

x 2 + y 2 − 14 x − 10 y − 151 = 0

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4-40

Analytical Geometry

C(7, 5)

r

P(2, −7)

Q

Centre is (7, 5) Radius = r = 49 + 25 + 151 = 15 The shortest distance of the point P from the circle = | CP − r |= = | CP − r |=

(2 − 7)2 + ( −7

(2 − 7)2 + ( −7 − 5)2 − 15



=| 13 − 15 |



= 2 units

Example 4.45 Let a, b and g  be the parametric angles of three points P, Q and R, respectively, on the circle x 2 + y 2 = a 2 and A be the point (−a, 0). If the length of the chords a b g AP, AQ and AR are in AP then show that cos , cos , cos are also in AP. 2 2 2

Solution LetP ( a cos a , a sin a ), Q( a cos b , a sin b ) , R( a cos r , a sin r ) A is (a, 0)

AP = ( a cos a + a)2 + a 2 sin 2 a = 4 a 2 cos 4 = 2a cos 2 Similarly AQ = 2a cos

a 2

a a  2a + sin 2  = 2a cos  cos 2 2 2

b 2



a a a + 4 a 2 sin 2 cos 2 2 2 2

AR = 2a cos

g 2

The lengths of chords AP, AQ, AR are in AP. a b u ∴ cos , cos , cos are also in AP. 2 2 2

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4-41

Circle

Example 4.46 Let S = x 2 + y 2 + 2 gx + 2 fy + c = 0 . Find the locus of the foot of the perpendicular from the origin on any chord of the circle that subtends a right angle at the origin.

Solution Let the equation of the line AB be lx + my = 1



(4.50)

Let (x1, y1) be the midpoint of AB. A

P(x1, y1)

B

O

Let P(x1, y1) be the foot of the perpendicular from the origin on AB. Then, since OP is perpendicular to AP.

−l y1 × = −1 ⇒ ly1 = mx1  m x1



l m 1 = = x1 y1 x12 + y12

(4.51)

Since (x1, y1) lies on the line lx + my = 1 we have lx1 + my1 = 1 



(4.52)

The combined equation of lines OA and OB is got by homogenizing the equation of the circle x 2 + y 2 + 2 gx + 2 fy + c = 0 with the line lx + my = 1. (i.e.) x 2 + y 2 + (2 gx + 2 fy )(lx + my ) + c(lx + my )2 = 0 (i.e.) x 2 (1 + 2 gl + cl 2 ) + y 2 (1 + 2 fm + cm2 ) + xy(2 gm + 2 f l + 2lmc) = 0

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4-42

Analytical Geometry

Since AOB = 90°, the condition is coefficient of x2 + coefficient of y = 0. Hence, 2



1 + 2 gl + cl 2 + 1 + 2 fm + cm2 = 0



2 2 (i.e.) c(l + m ) + (2 gl + 2 fm) + 2 = 0



  x12  x   y  y12 c 2 + + 2g  2 1 2  + 2 f  2 1 2  + 2 = 0 2 2 2 2 2  x1 + y1   x1 + y1   ( x1 + y1 ) ( x1 + y1 ) 



(i.e.)

2 gx 2 fy c + 2 12 + 2 1 2 +2= 0 2 x + y1 x1 + y1 x1 + y1 2 1

or 2( x12 + y12 ) + 2 gx1 + 2 fy1 + c = 0



The locus of (x1, y1) is 2( x 2 + y 2 ) + 2 gx + 2 fy + c = 0.

Example 4.47 P is the point (a, b) and Q is the point (b, a). Find the equation of the circle touching OP and OQ at P and Q where O is the origin.

Solution Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + c = 0  Let C(−g, −f  ) be the centre of the circle.

(4.53)

P(a, b)

O

C

Q(b, a)

We know that PQ is the chord of the contact from O and OC is perpendicular to PQ. \ Slope of PQ × slope of OC = −1

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f ( b − a) =− g ( a − b) ∴f =g

(4.54)

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Circle

4-43

y b = or bx − ay = 0. x a Since CP is perpendicular to OP, r is the perpendicular distance from C on OP.

The equation of OP is

g2 + f 2 − c = ±



( − gb + af ) a2 + b2

( g 2 + f 2 − c)( a 2 + b 2 ) = (bg − af )2

(i.e.)

(2 g 2 − c)( a 2 + b 2 ) = g 2 (b − a)2

or

g 2 2( a 2 + b 2 ) − (b − a)2  = c( a 2 + b 2 )



⇒c=



g 2 ( a + b) 2 a2 + b2

The point (a, b) lies on the circle (4.53). a 2 + b 2 + 2 ga + 2 f b + c = 0



a 2 + b 2 + 2 g ( a + b) +



g 2 ( a + b) 2 =0 a2 + b2



( a + b) 2 g 2 + 2 g ( a + b) + ( a 2 + b 2 ) 2 = 0



 g ( a + b) + ( a 2 + b 2 )  = 0

2

⇒g=

−( a 2 + b 2 ) a+b

Hence the equation of the circle is

x2 + y2 − 2

( a2 + b2 ) ( a2 + b2 ) ( a + b) 2 ( a 2 + b 2 ) 2 x−2 y+ 2 =0 ⋅ a+b a+b a + b 2 ( a + b) 2

2 2 2 2 2 2 (i.e.)  ( a + b)( x + y ) − 2( a + b )( x + y ) + ( a + b)( a + b ) = 0

Example 4.48 A circle of circumradius 3k passes through the origin and meets the axes at A and B. Show that the locus of the centroid of ∆OAB is the circle x2 + y2 = 4 K 2 .

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4-44

Analytical Geometry

Solution Let A and B be the points (a, 0) and (0, b), respectively. Let (x1, y1) be the centroid of ∆OAB. Then since AOB = 90°, AB is a diameter of the circle. y B(0, b)

O

A(a, 0)

x

2 2 2 (4.55) ∴ OA2 + OB 2 = (6k )2 or a + b = 36k  a b Let the centroid of ∆OAB be (x1, y1). Then x1 = and y1 = or a = 3 x1 and 3 3 b = 3 y1 . Substituting this in (4.55), we get 9 x12 + 9 y12 = 36k 2 or x12 + y12 = 4 k 2.



The locus of (x1, y1) is x 2 + y 2 = 4 k 2.

Example 4.49 A variable line passes through a fixed point (a, b) and cuts the coordinate axes at the points A and B. Show that the locus of the centre of the circle AB a b is + = 2. x y

Solution Let AB be a variable line whose equation be

x y + = 1. a b

y B(0, b ) b A(a, 0) O

a

x

This passes through the point (a, b). a b ∴ + =1 a b

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Circle

4-45

Since AOB = 90°, AB is a diameter of the circumcircle of ∆OAB. Its centre is a b a b  = x1 and = y1 .  ,  . If (x1, y1) be the circumcentre, then 2 2 2 2 ∴ a = 2 x1 and b = 2 y1



Hence, from (4.55), we get a b + =1 2 x1 2 y1

The locus of (x1, y1) is

a b + = 2. x y

Example 4.50 If 4l 2 − 5m2 + 6l + 1 = 0 then show that the line lx + my + 1 = 0 touches a fixed circle. Find the centre and radius of the circle.

Solution Let the line

lx + my + 1 = 0 

(4.56)

( x − h)2 + ( y − k )2 = r 2 

(4.57)

touch the circle

Then the perpendicular distance from (h, k) to line (4.56) is equal to the­ radius. ∴



lh + mk + 1 l 2 + m2

= ±a

(i.e.) a 2 (l 2 + m2 ) = (lh + mk + 1)2 or l 2 [a 2 − h2 ] + m2 [a 2 − k 2 ] −2lmhk − 2lh + 2mk − 1 = 0

(4.58)

But the condition is given by 4l 2 − 5m2 + 6l + 1 = 0 



(4.59)

Identifying (4.58) and (4.59), we get

a 2 − h2 a 2 − k 2 2hk −2h 2k −1 = = = = = 4 −5 0 6 0 1

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 45

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4-46

Analytical Geometry

a 2 − h2 = −1 4 ⇒ a 2 − h2 = −4 ⇒ a2 = 9 − 4 = 5

∴ k = 0, h = 3 and



Hence, the line touches the fixed circle ( x − 3)2 + y 2 = 5 or x 2 + y 2 − 6 x + 4 = 0 whose centre is (3, 0) and radius is 5.

Exercises 1.

Find the equation of the following circles: (i) centre (2, −5) and radius 5 units Ans.: (i) x 2 + y 2 − 4 x + 10 y + 4 = 0 (ii) centre (−2, −4) and radius 10 units Ans.: (ii) x 2 + y 2 + 4 x + 8 y − 80 = 0 (iii) centre (a, b) and radius (a + b) Ans.: (iii) x 2 + y 2 − 2ax − 2by = 0

2.

Find the centre and radius of the following circles: (i) x 2 + y 2 − 22 x − 4 y + 25 = 0 

3.

Ans.: (i) (11, 2), 10 1 Ans.: (ii) (1, -2), (ii) 4( x + y ) − 8( x − 2 y ) + 19 = 0  2  −3 1  1 Ans.: (iii)  , −  , (iii) 2 x 2 + 2 y 2 + 3 x + y + 1 = 0   4 4 2 2 Find the equation of the circle passing through the point (2, 4) and having its centre on the lines x − y = 4 and 2 x + 3 y = 8. 2

2

Ans.: x 2 + y 2 − 8 x − 4 = 0

 4.

Find the equation of the circle whose centre is (−2, 3) and which passes through the point (2, −2). Ans.: x 2 + y 2 + 4 x − 6 y − 28 = 0

 5.

Show that the line 4 x − y = 17 is a diameter of the circle x 2 + y 2 − 8 x + 2 y = 0.

6.

The equation of the circle is x 2 + y 2 − 8 x + 6 y − 3 = 0. Find the equation of its diameter parallel to 2 x − 7 y = 0 . Also find the equation of the diameter perpendicular to 3 x − 4 y + 1 = 0. Ans.: 2 x − 7 y − 29 = 0 4x + 3y − 7 = 0

  7.

Find the equation of the circle passing through the following points: (i) (2, 1), (1, 2), (8, 9) Ans.: x 2 + y 2 − 10 x − 10 y − 25 = 0 (ii) (0, 1), (2, 3), (−2, 5) Ans.: 3 x 2 + 3 y 2 + 2 x − 20 y + 17 = 0 (iii) (5, 2), (2, 1), (1, 4) Ans.: x 2 + y 2 − 6 x − 6 y + 13 = 0

8.

Find the equation of the circle through the points (1, 0) and (0, 1) and having its centre on the line x + y = 1.



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Ans.: x 2 + y 2 − x − y = 0

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Circle

9.

Find the equation of the circle passing through the points (0, 1) and (4, 3) and having its centre on the line 4 x − 5 y − 5 = 0. Ans.: x 2 + y 2 − 5 x − 2 y + 1 = 0

 10.

Two diameters of a circle are 5 x − y = 3 and 2 x + 3 y = 8 . The circle passes through the point (−1, 7). Find its equation. Ans.: x 2 + y 2 − 2 x − 4 y = 164

 11.

4-47

Find the equation of the circle circumscribing the triangle formed by the axes and the straight line 3 x + 4 y + 12 = 0. Ans.: x 2 + y 2 + 4 x + 3 y = 0

 12.

Show that the points (−1, 2), (−2, 4), (−1, 3) and (2, 0) are on a circle and find its equation.

13.

If the coordinates of the extremities of the diameter of a circle are (3, 5) and (−7, −5), find the equation of the circle. Ans.: x 2 + y 2 + 4 x − 3 y = 0

 14.

Find the equation of the circle when the coordinates of the extremities of one of its diameters are (4, 1) and (−2, −7). Ans.: x 2 + y 2 − 2 x + 6 y − 15 = 0

 15.

If one end of the diameter of the circle x 2 + y 2 − 2 x + 6 y − 15 = 0 is (4, 1), find the coordinates of the other end. Ans.: (−2, −7)

 16.

Prove that the tangents from (0, 5) to the circles x + y + 2 x − 4 = 0 and x 2 + y 2 − y + 1 = 0 are equal.

17.

Find the equation of the circle passing through the origin and having its centre at (3, 4). Also find the equation of the tangent to the circle at the origin.

2

Ans.: x 2 + y 2 − 6 x − 8 y = 0, 3 x + 4 y = 0

 18.

 19.

2

Find the slope of the radius of the circle x 2 + y 2 = 25 through the point (3, −4) and hence write down the equation of the tangent to the circle at the point. What are the intercepts made by this tangent on the x-axis and y-axis? −4 25 −25 Ans.: , 3 x − 4 y = 25, , 5 3 4 One vertex of a square is the origin and two others are (4, 0) and (0, 4). Find the equation of the circle circumscribing the square. Also find the equation of the tangent to this circle at the origin.



M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 47

Ans.: x 2 + y 2 − 4 x − 4 = 0, x + y = 0

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4-48

20.

Analytical Geometry

A circle passes through the origin and the points (6, 0) and (0, 8). Find its equation and also the equation of the tangent to the circle at the origin. Ans.: x 2 + y 2 − 6 x − 8 y = 0, 3 x + 4 y = 0

 21.

A and B are two fixed points on a plane and the point P moves on the plane in such a way that PA = 2PB always. Prove analytically that the locus of P is a circle.

22.

Does the point (2, 1) lie (i) on, (ii) inside or (iii) outside the circle x2 + y2 − 4 x − 6 y + 9 = 0 ?

23.

Show that the circles x 2 + y 2 − 2 x + 2 y + 1 = 0 and x 2 + y 2 + 6 x − 4 y − 3 = 0 touch each other externally.

24.

Prove that the centres of the three circles x 2 + y 2 − 2 x + 6 y + 1 = 0 , x 2 + y 2 + 4 x − 12 y − 9 = 0 and x 2 + y 2 = 25 lie on the same straight line. What is the equation of this line? Ans.: 3 x + 4 y = 0

 25.

Prove that the two circles x 2 + y 2 + 2ax + c 2 = 0 and x 2 + y 2 + 2by + c 2 = 0 1 1 1 touch each other if 2 + 2 = 2 . a b c

26.

Show that the circles x 2 + y 2 − 4 x + 2 y + 1 = 0 and x 2 + y 2 − 12 x + 8 y + 43 = 0 touch each other externally.

27.

Show that the circles x 2 + y 2 = 400 and x 2 + y 2 − 10 x − 24 y + 120 = 0 touch one another. Find the co-ordinates of the point of contact.  100 240  Ans.:  ,  13 13  Find the length of the tangent from the origin to the circle 4 x 2 + 4 y 2 + 6 x + 7 y + 1 = 0.

 28. 29.

Show that the circles x + y − 26 x − 19 = 0 and x 2 + y 2 + 3 x − 8 y − 43 = 0 touch externally. Find the point of contact and the common tangent.

30.

A point moves so that the square of its distance from the base of an isosceles triangle is equal to the rectangle contained by its distances from the equal sides. Prove that the locus is a circle.

31.

Prove that the centres of the circles x 2 + y 2 = 1, x 2 + y 2 + 4 x + 8 y − 1 = 0 and x 2 + y 2 − 6 x − 12 y + 1 = 0 are collinear.

32.

Prove that the constant in the equation of the circle x 2 + y 2 + 2 gx + 2 fy + c = 0 is equal to the rectangle under the segments of the chords through the origin.

33.

Find the equation of the locus of a point that moves in a plane so that the sum of the squares from the line 7 x − 4 y − 10 = 0 and 4 x + 7 y + 5 = 0 is always equal to 3.

2



M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 48

2

Ans.: 13 x 2 + 13 y 2 − 20 x + 30 y − 14 = 0

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4-49

Circle

34.

 35.

Show that the circles x 2 + y 2 −10 x + 4 y − 20 = 0 and x 2 + y 2 + 14 x − 6 y + 22 = 0 touch each other. Find the equation of their common tangent at the point of contact and also the point of contact. −14 9  Ans.:  , 12 x − 5 y + 21 = 0 ,  13 13  L and M are the feet of the perpendicular from (c, 0) on the lines ax 2 + 2hxy + by 2 = 0 . Show that the equation of LM is ( a + b) x + 2hy + bc = 0.

36.

A circle has radius 3 units and its centre lies on the line y = x −1. Find the equation of the circle if it passes through (−1, 3).

37.

Find the equation of the circle on the line joining the points (−4, 3) and (12, −1). Find also the intercepts made by it on the y-axis.



Ans.: x 2 + y 2 − 8 x − 2 y − 51 = 0; 4 13.  11 + 6  Show that the points 1, lies outside the circle 3 x 2 + 3 y 2 − 5 x − 6 y + 4 = 0. 6  

38.

3 x 2 + 3 y 2 − 5 x − 6 y + 4 = 0. 39.  40.

Find the condition that the line lx + my + n = 0 touches the circle x 2 + y 2 = a 2 . Find also the point of contact.  − a 2l − a 2 m  , Ans.:  n   n Find the equation of the circle passing through the point (3, 5) and (5, 3) and having its centre on the line 2 x + 3 y − 1 = 0. Ans.: 5 x 2 + 3 y 2 − 14 x − 14 y − 50 = 0

 41.

ABCD is a square whose side is a. Taking line AO as the axis of coordinates, prove that the equation of the circumcircle of the square is

x 2 + y 2 − ax − ay = 0.

42.

Find the equation of the circle with its centre on the line 2 x + y = 0 and touching the lines 4 x − 3 y + 10 = 0 and 4 x − 3 y − 3 = 0. Ans.: x 2 + y 2 − 2 x + 4 y − 11 = 0

 43.

Find the equation of the circle that passes through the point (1, 1) and touches the circle x 2 + y 2 + 4 x − 6 y − 3 = 0 at the point (2, 3) on it. Ans.: x 2 + y 2 + x − 6 y + 3 = 0

 44.

2 2 Prove that the tangent to the circle x + y = 5 at the point (1, −2) also 2 2 touches the circle x + y − 8 x + 6 y + 20 = 0 and find its point of contact.

Ans.: (3, −1)

 45.

A variable circle passes through the point A(a, b) and touches the x-axis. Show 2 that the locus of the other end of the diameter through A is ( x − c ) = 4by.

46.

Find the equation of the circle passing through the points A(−5, 0), B(1, 0), and C(2, 1) and show that the line 4 x − 3 y − 5 = 0 is a tangent to the line.

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4-50

47.

Analytical Geometry

Find the equation of the circle through the origin and through the point of contact of the tangents from the origin to the circle. Ans.: 2 x 2 + 2 y 2 − 11x − 13 y = 0

 48.

The circle x 2 + y 2 − 4 x − 4 y + 4 = 0 is inscribed in a triangle that has two of its sides along the coordinate axes. The locus of the circumference of the triangle is x + y − xy + k x 2 + y 2 = 0. Find k. Ans.: k = 1

 49.

A circle of diameter 13 m with centre O coinciding with the origin of coordinate axes has diameter AB on the x-axis. If the length of the chord AC be 5 m, find the area of the smaller portion bounded between the circles and the chord AC. Ans.:1.9 m2.

 50.  51.

Find the radius of the smallest circle that touches the straight line 3 x − y = 6 at (1, −3) and also touches the line y = x.  1 If  mi , m 

i

Ans.: 10 2 − 4 10   , mi > 0, i = 1, 2, 3, 4 form distinct points on a circle show that m1,

m2, m3, m4 = 1.

If the line x cos a + y sin a = ρ cuts the circle x 2 + y 2 = a 2 in M and N, then show that the circle whose diameter is MN is x 2 + y 2 − a 2 − 2 r (x cos a + y sin a − r ) = 0. 2 2 2 + y − a − 2 r (x cos a + y sin a − r ) = 0. 52.

53.

Show that the tangents drawn from the point (8, 1) to the circle x 2 + y 2 − 2 x − 4 y − 20 = 0 are perpendicular to each other.

54.

How many circles can be drawn each touching all the three lines x + y = 1, y = x + 1 and 7 x − y = 6 ? Find the centre and radius of all the circles. 7  7 2 Ans.: 4;  , 1 , 2  4

 55.  56.  57.

Find the points on the line x − y + 1 = 0, the tangents from which to the circle x 2 + y 2 − 3 x = 0 are of length 2 units. 3 5 Ans.:  ,  ; (−1, 0)  2 2 On the circle 16 x 2 + 16 y 2 + 48 x − 3 y − 43 = 0, find the point nearest to the line 8 x − 4 y + 73 = 0 and calculate the distance between this point and the line.  −7 5  Ans.:  ,  ; 2 5  2 4 Find the equations of the lines touching the circle x 2 + y 2 + 10 x − 2 y + 6 = 0 and parallel to the line 2 x + y − 7 = 0.



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Ans.: 2 x + y − 1 = 0 , 2 x + y + 19 = 0

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4-51

Circle

58.

Find the equation of the circle whose diameter is the chord of intersection of the line x + 3 y = 6 and the curve 4 x 2 + 9 y 2 = 36. Ans.: 5( x 2 + y 2 ) − 12 x − 16 y + 12 = 0

 59.

Find the equation for the circle concentric with the circle x 2 + y 2 − 8 x + 6 y − 5 = 0 and passes through the point (−2, 7). Ans.: x 2 + y 2 − 8 x + 6 y − 27 = 0

 60.

Find the equation of the circle that cuts off intercepts −1 and −3 on the x-axis and touches the y-axis at the point 0, 3 .

)

(

Ans.: x 2 + y 2 + 4 x − 2 3 y + 3 = 0

 61.

 62.  63.

Find the coordinates of the point of intersection of the line 5 x − y + 7 = 0 and the circle x 2 + y 2 + 3 x − 4 y − 9 = 0. Also find the length of the common segment. 117  −1 −1 Ans.:  ,  and (1, 7),  2 2 2 The line 4 x + 3 y + k = 0 is a tangent to the circle x 2 + y 2 = 4 . Find the value of k. Ans.: k = ±10 Find the equations of tangents to the circle x 2 + y 2 − 6 x + 4 y − 17 = 0 that are perpendicular to 3 x − 4 y + 5 = 0. Ans.: 4 x + 3 y + 19 = 0, 4 x + 3 y − 31 = 0

 64.

Find the equation of tangents to the circle x 2 + y 2 − 14 x + y − 5 = 0 at the points whose abscissa is 10. Ans.: 3 x + 7 y − 93 = 0, 3 x − 7 y − 64 = 0

 65.

Show that the circles x 2 + y 2 − 4 x + 6 y + 8 = 0 and x 2 + y 2 − 10 x − 6 y + 14 = 0 touch each other. Find the point of contact. Ans.: (3, −1)

 66.

Show that the tangent to the centre x + y = 0 at the point (1, −2) also touches the circle x 2 + y 2 − 8 x + 6 y + 20 = 0. Find the point of contact. 2

2

Ans.: (3, −1)

 67.

A straight line AB is divided at C so that AB = 3CB. Circles are described on AC and CB as diameters and a common tangent meets AB produced at D. Show that BD is equal to the radius of the smaller circle.

68.

The lines 3 x − 4 y + 4 = 0 and 6 x − 8 y − 7 = 0 are tangents to the same circle. Find the radius of this circle.



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Ans.:

3 4

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4-52

69.

Analytical Geometry

From the origin, chords are drawn to the circle ( x − 1)2 + y 2 = 1. Find the equation of the locus of the midpoint of these chords. Ans.: x 2 + y 2 − x = 0

 70.

Find the equations of the pair of tangents to the circle x 2 + y 2 − 2 x + 4 y = 0 from (0,1). Ans.: 2 x 2 − 2 y 2 + 3 xy − 3 x + 4 y − 2 = 0

 71.

If the polar of points on the circle x 2 + y 2 = a 2 with respect to the circle x 2 + y 2 = b 2 touch the circle x 2 + y 2 = c 2 , show that a, b and c are in GP.

72.

If the distances of origin to the centres of three circles x 2 + y 2 − 2lx = c 2 where l is a variable and c is a constant are in G. P, prove that the length of the tangent drawn to them from any point on the circle x 2 + y 2 = c 2 are in G. P.

73.

A tangent is drawn to each of the circles x 2 + y 2 = a 2 and x 2 + y 2 = b 2 . If the two tangents are mutually perpendicular, show that the locus of their point of intersection is a circle concentric with the given circles.

74.

If the pole of any line with respect to the circle x 2 + y 2 = a 2 lies on the circle x 2 + y 2 = 9a 2, then show that the line will be a tangent to the circle a2 x2 + y2 = . 9 A triangle has two of its sides along the y-axis, and its third side touches the circle x 2 + y 2 − 2ax − 2ay + a 2 = 0. Prove that the locus of the circumcentre of the triangle is 2 xy − 2a( x + y ) + a 2 = 0.

75.

76.

Lines 5 x + 12 y − 10 = 0 and 6 x − 11 y − 40 = 0 touch a circle C, of diameter 6. If the centre of C1 lies in the first quadrant, find the equation of circle C2. which is concentric with C1 and cuts intercepts of length 8 on these lines. 2

 77.

Ans.: x 2 + y 2 + 10 x − 8 y + 16 = 0

 78.

2

65   5  Ans.:  x −  +  y +  = 25  5  4 Find the equation of the circle that touches the y-axis at a distance of 4 units from the origin and cuts off an intercept of 6 units from the x-axis. Find the equation of the circle in which the line joining the points (0, b) and (b, −a) is a chord subtending an angle 45° at any point on its circumference Ans.: x 2 + y 2 − 2( a + b) x + 2( a − b) y + ( a 2 + b 2 )

 79.

From any point on a given circle, tangents are drawn to another circle. Prove that the locus of the middle point of the chord of contact is a third circle; the distance between the centres of the given circle is greater than the sum of their radii.

80.

A point moves so that the sum of the squares of the perpendiculars that fall from it on the sides of an equilateral triangle is constant. Prove that the locus is a circle.

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4-53

Circle

81.

A circle of constant radius passes through the origin O and cuts the axes in A and B. Show that the locus of the foot of the perpendicular from AB is ( x 2 + y 2 )2 ( x 2 + y 2 ) = 4 r 2 .

82.

Find the equation of the image of the circle ( x − 3)2 + ( y − 2)2 = 1 by the mirror x + y = 19. Ans.: ( x − 17)2 + ( y − 16)2 = 1

 83.  84.

85.

Find the value of l for which the circle x 2 + y 2 + 6 x + 5 + l( x 2 + y 2 − 8 x + 7) = 0 dwindles into a point.

4 2 3 A variable circle always touches the line y = x and passes through the point (0, 0). Show that the common chords of this circle and  1 1 x 2 + y 2 + 6 x + 8 y − 7 = 0 will pass through a fixed point  ,  .  2 2 The equation of the circle that touches the axes of the coordinates and the line x y + = 1 and whose centre lies in the first quadrant is 3 4 2 x + y 2 − 2cx − 2cy + c 2 = 0. Find the values of c. Ans.: 2 ±

Ans.: (1, 6)

 86.

A region in xy-plane is bounded by the curve y = 25 − x and the line y = 0. If the point ( a, a + 1) lies in the interior of the region, find the range of a. 2

Ans.: a¨(−1, 3)

 87.

The points (4, −2) and (3, 6) are conjugate with respect to the circle x 2 + y 2 = 24. Find the value of b. Ans.: b = −6

 88.

If the two circles x + y + 2 gx + 2 fy = 0 and x + y + 2 g x + 2 f 1 y = 0 touch each other, show that f 1 g = gf 1.

89.

Show that the locus of the points of chords of contact of tangents subtending a right angle at the centre is a concentric circle whose radius is 2 times the radius of the given circle. Also show that this is also the locus of the point of intersection of perpendicular tangents.

90.

Show that the points (xi, yi), i = 1, 2, 3 are collinear if and only if their poles with respect to the circles x 2 + y 2 = a 2 are concurrent.

91.

The length of the tangents from two given points A and B to a circle are t1 and t2, respectively. If the points are conjugate points, show that AO 2 = t12 + t22 .

92.

Show that the equation to the pair of tangents drawn from the origin to the circle x 2 + y 2 + 2 gx + 2 fy + c = 0 is ( gx + fy )2 = ( f 2 + g 2 ). Hence find the locus of the centre of the circle if these tangents are perpendicular.

2

2



M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 53

2

2

1

Ans.: x 2 + y 2 = 2c

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4-54

Analytical Geometry

  93. Three sides of a triangle have the equations Li = y − mr x − cr = 0, r = 1, 2, 3. Then show that lL2 L3 + mL3 L1 + uL1 L2 = 0 where l, m, u ó 0 is the equation of the circumcircle of the triangle if ∑ l( m2 + m3 ) = 0 and ∑ l(m2 m3 − m1 ) = 0.   94. A triangle is formed by the lines whose combined equation is c( x + y − 4)( xy − 2 x − y + 2) = 0. Show that the equation of its circumference is x 2 + y 2 − 3 x − 5 y + 8 = 0.   95. Two distinct chords drawn from the point (p, q) on the circle x 2 + y 2 = px + qy, where pq ≠ 0, are bisected by the x-axis. Show that p 2 > 8q 2 .   96. Show that the number of points with integral coordinates that are interior to the circle x 2 + y 2 = 16 is 45.   97. Find the number of common tangents to the circles x 2 + y 2 − 6 x − 14 y + 48 = 0 and x 2 + y 2 − 6 x = 0. Ans.: 4



  98. The tangents to the circle x 2 + y 2 = 4 at the points A and B meet at P(-4, 0). Find the area of the quadrilateral PAOB. Ans.: 4 3 m2



  99. The equations of four circles are ( x ± a ) + ( y ± a ) = a 2 . Find the radius of a circle touching all the four circles. 2

2

Ans.:



(

)

2 ±1 a

100. A circle of radius 2 touches the coordinate axes in the first quadrant. If the circle makes a complete rotation on the x-axis along the positive direction of the x-axis, then show that the equation of the circle in the new position is x 2 + y 2 − 4( x + y ) − 8lx + (2 + 4p )2 = 0. 101. Two tangents are drawn from the origin to a circle with centre at (2, −1). If the equation of one of the tangents is 3 x + y = 0, find the equation of the other tangent. Ans.: x − 3 y = 0



102. Find the equation of the chord of the circle x + y = a passing through the point (2, 3) farther from the centre. 2



2

2

Ans.: 2 x + 3 y = 17

103. An equilateral triangle is inscribed in the circle x 2 + y 2 = a 2, with the vertex at (a, 0). Find the equation of the side opposite to this vertex. 

Ans.: 2 x + a = 0

104. A line is drawn through the point P(3, 1) to cut the circle x 2 + y 2 = 9 at A and B. Find the value of PA ⋅ PB. 

M04_ANALYTICALGEOMETRY_ISBN_CH04.indd 54

Ans.: 121

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Circle

4-55

105. C1 and C2 are circles of unit radius with their centres at (0, 0) and (1, 0), respectively. C3 is a circle of unit radius, passing through the centres of the circles C1 and C2 and having its centre above the x-axis. Find the equation of the common tangent to C1 and C3 that passes through C2. Ans.: 3 x − y + 2 = 0



106. A chord of the circle x + y − 4 x − 6 y = 0 passing through the origin  7 subtends an angle tan −1   at the point where the circle meets the positive  4 y-axis. Find the equation of the chord. 2

2

Ans.: x − 2 y = 0



107. A circle with its centre at the origin and radius equal to a meets the axis of x at A and B. P and Q are respectively the points ( a cos a , a tan a ) and ( a cos b , a tan b ) such that a − b = 2g . Show that the locus of the point of intersection of AP and BQ is x 2 + y 2 − 2ay tang = −2 . 108. A circle C1 of radius touches the circle x 2 + y 2 = a 2 externally and has its centre on the positive x-axis. Another circle C2 of radius c touches circle C1 externally and has its centre on the positive x-axis. If a < b < c, show that the three circles have a common tangent if a, b, c are in GP. 109. Find the equations of common tangents to the circles x 2 + y 2 + 14 x − 14 y + 28 = 0 and x 2 + y 2 − 14 x + 4 y − 28 = 0 

Ans.: 28 y + 45 y + 371 = 0 and y − 7 = 0.

110. If a circle passes through the points of intersection of the coordinate axes with the line x − ly + 1 = 0 ( l ≠ 0 ) and x − 2 y + 3 = 0 then l satisfies the equation 6l 2 − 7l + 2 = 0. 111. OA and OB are equal chords of the circle x 2 + y 2 − 2 x + 4 y = 0 perpendicular to each other and passing through the origin. Show that the slopes of OA and OB satisfy the equation 3m2 − 8m − 3 = 0 . 112. Find the equation of the circle passing through the points (1, 0) and (0, 1) and having the smallest possible radius. 

Ans.: x 2 + y 2 − x − y = 0

113. Find the equation of the circle situated systematically opposite to the circle x 2 + y 2 − 2 x = 0 with respect to the line x + y = 2. 

Ans.: x 2 + y 2 − 4 x − 2 y + 4 = 0

114. O is a fixed point and R moves along a fixed line L not passing through O. If S is taken on OR such that OR ⋅ OS = K 2 , then show that the locus of S is a circle. 115. Show that the circumference of the triangle formed by the lines ax + by + c = 0, bx + cy + a = 0 and cx + ay + b = 0 passes through the origin if (b 2 + c 2 )(c 2 + a 2 )( a 2 + b 2 ) = abc(b + c)(c + a)( a + b).

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4-56

Analytical Geometry

116. Two circles are drawn through the points (a, 5a) and (4a, a) to touch the  40  y-axis. Prove that they intersect at an angle tan −1   .  9 117. Show that the locus of a point P that moves so that its distance from the given point O is always in a given ratio n : 1 ⋅ ( n ≠ −1) to its distance on the line joining the points that divides the line OA in the given ratio as diameter. 118. The lines 3 x − 4 y + 4 = 0 and 6 x − 3 y − 7 = 0 are tangents to the same circle. Find the radius of the circle. 3  Ans.: r = 4 119. The line y = x touches a circle at P so that OP = 4 2 where O is the origin. The point (−10, 2) is inside the circle and length of the chord on the line x + y = 0 is 6 2. Find the equation of the line. Ans.: x 2 + y 2 + 18 x − 2 y + 32 = 0



120. Find the intervals of values of a for which the line y + x = 0 bisects two  1 + 2a 1 − 2a  chords drawn from a point  , to the circle 2   2

(

) (

)

2 x 2 + 2 y 2 − 1 + 2a x − 1 − 2a y = 0 . 121. Show that all chords of the circle 3 x 2 − y 2 − 2 x + 4 y = 0 that subtend a right angle at the origin are concurrent. Does the result hold for the curve 3 x 2 + 3 y 2 − 2 x + 4 y = 0 ? If yes, what is the point of concurrency, and if not, give the reason. 122. Find the equations of the common tangents to the circles x 2 + y 2 − 14 x + 6 y + 33 = 0 and x 2 + y 2 + 30 x − 20 y + 1 = 0. 

Ans.: 4 x − 3 y − 12 = 0, 24 x + 7 y − 22 = 0

123. Prove that the orthocentre of the triangle whose angular points are ( a cos a , a sin a ), ( a cos b , a sin b ) and ( a cos g , a sin g ) is the point [a(cos a + cos b + cosg ), a(sin a + sin b + sin g )].

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Chapter

5

System of Circles

5.1  RADICAL AXIS OF TWO CIRCLES Definition 5.1.1: The radical axis of two circles is defined as the locus of a point such that the lengths of tangents from it to the two circles are equal. Obtain the equation of the radical axis of the two circles S ≡ x2 + y2 + 2gx + 2fy + c = 0 and S1 ≡ x2 + y2 + 2g1x + 2f1 y + c1 = 0. P

T1

T

A

B

Let P(x1, y1) be a point such that the lengths of tangents to the two circles are equal.

(i.e.)  PT = PT1 ⇒ PT 2 = PT12 ∴x12 + y12 + 2gx1 + 2fy1 + c = x12 + y12 + 2g1x1 + 2f1y1 + c1 2(g - g1)x1 + 2( f - f1)y1 + c - c1 = 0

The locus of (x1, y1) is 2(g - g1)x + 2( f - f1)y + (c - c1) = 0 which is a straight line. Therefore, the radical axis of two given circle is a straight line.

Note 5.1.1:  If S = 0 and S1 = 0 are the equations of two circles with unit coefficients for x2 and y2 terms then the equation of the radical axis is S - S1 = 0.

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5-2

Analytical Geometry

Note 5.1.2:  Radical axis of two circles is a straight line perpendicular to the line of centres. The centres of the two circles are A(−g, −f ) and B(−g1, −f1). − f + f1 f − f1 The slope of the line of centres is m1 = = − g + g1 g − g1 −( g − g1 ) The slope of the radical axis is m2 = f1 − f1

∴ m1m2 = −1

Therefore, the radical axis is perpendicular to the line of centres.

Note 5.1.3:  If the two circles S = 0 and S1 = 0 intersect then the radical axis is the common chord of the two circles.

Note 5.1.4:  If the two circles touch each other, then the radical axis is the common tangent to the circles. Note 5.1.5:  If a circle bisects the circumference of another circle then the radical axis passes through the centre of the second circle. Show that the radical axes of three circles taken two by two are concurrent. Let S1 = 0, S2 = 0 and S3 = 0 be the equations of three circles with unit coefficients for x2 and y2 terms. Then the radical axes of the circles taken two by two are S1 - S2 = 0, S2 - S3 = 0 and S3 - S1 = 0. \ (S1 - S2) + (S2 - S3) + (S3 - S1) ≡ 0 Since sum of the terms vanishes identically, the lines represented by S1 - S2 = 0, S2 - S3 = 0 and S3 - S1 = 0 are concurrent. The common point of the lines is called the radical centre.

5.2  ORTHOGONAL CIRCLES Definition 5.2.1: Two circles are defined to be orthogonal if the tangents at their point of intersection are at right angles. Find the condition for the circles S ≡ x2 + y2 + 2gx + 2fy + c = 0, S1 ≡ x2 + y2 + 2g1x + 2f1 y + c1 = 0 to be orthogonal. P r1 A

M05_ANALYTICALGEOMETRY_ISBN_CH05.indd 2

r2 B

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System of Circles

5-3

Let P be a point of intersection of the two circles S = 0 and S1 = 0. The centres are A(−g, −f ), B(−g1, −f1). The radii are r1 =

g 2 + f 2 − c and r2 =

g 22 + f 22 − c1 .

Since the two circles are orthogonal, PA is perpendicular to PB. (i.e.)  APB is a right triangle.

\ AB2 = PA2 + PB2



⇒(−g + g1)2 + (−f + f1)2 = (g2 + f 2 - c) + (g12 + f 12 - c1)



⇒g2 + g12 - 2gg1 + f 2 + f 12 – 2ff1 = g2 + g12 + f 2 + f 12 - c - c1



⇒2gg1 + 2ff1 = c + c1

Show that if a circle cuts two given circles orthogonally then its centre lies on the radical axis of the two given circles. Let S1 = x2 + y2 + 2g1x + 2f1 y + c1 = 0 and S2 = x2 + y2 + 2g2 x + 2f2 y + c1 = 0 be the two given circles. Let S = x2 + y2 + 2gx + 2fy + c = 0 cuts S1 = 0 and S2 = 0 orthogonally. Since S = 0 cuts S1 = 0 and S2 = 0 orthogonally,

2gg1 + 2ff1 = c + c1;

2gg2 + 2ff2 = c + c2

By subtracting, we get 2g(g1 - g2) + 2f( f1 - f2) = c1 - c2

or

−2g(g1 - g2) - 2f( f1 - f2) + (c1 - c2) = 0

This shows that (−g, −f ) lies on the line, 2(g1 - g2)x + 2( f1 - f2)y + (c1 - c2) = 0 which is the radical axis of the two circles. Therefore, the centre of the circle S = 0 lies on the radical axis of the circles S1 = 0 and S2 = 0.

5.3  COAXAL SYSTEM Definition 5.3.1:  A system of circles is said to be coaxal if every pair of the system has the same radical axis. Express the equation of a coaxal system of circles in the simplest form. In a coaxal system of circles, every pair of the system has the same radical axis. Therefore, there is a common radical axis to a coaxal system of circles. Hence, in a coaxal system the centres are all collinear and the common radical axis is perpendicular to the lines of centres. Therefore, let us choose the line of centres as x-axis and the common radical axis as y-axis.

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5-4

Analytical Geometry

Let us consider two circles of the coaxal system,

x2 + y2 + 2gx + 2fy + c = 0, x2 + y2 + 2g1x + 2f1y + c1 = 0

Since the centres lie on the x-axis, f1 = 0 and f2 = 0. Therefore, the equations of the circles are x2 + y2 + 2gx + c = 0 and x2 + y2 + 2g1x + c1 = 0. The radical axis of these two circles is 2(g - g1)x + (c - c1) = 0. However, the common radical axis is the y-axis whose equation is x = 0.

∴c - c1 = 0  or  c = c1.

Hence, the general equation to a coaxal system of circles is x2 + y2 + 2gx + c = 0 where g is a variable and c is a constant. So the equation of a coaxal system can be expressed in the simplest form x2 + y2 + 2l x + c = 0



where l is a variable and c is a constant.

5.4  LIMITING POINTS Definition 5.4.1:  Limiting points are defined to be the centres of point circles belonging to a coaxal system; that is, they are centres of circles of zero radii belonging to a coaxal system. Obtain the limiting points of the coaxal system of circles x2 + y2 + 2lx + c = 0. Centres are (−l, 0) and radii are For point circles radii are zero.

k 2 - c.

∴ l 2 − c = 0 or l 2 = c



∴ l = ( ± c , 0)

Therefore, limiting points are ( c , 0) and ( − c , 0) .

Theorem 5.4.1:  The polar of one limiting point of a coaxal system of circles with respect to any circle of the system passes through the other limiting point.

Proof:  Let x-axis be the line of centres and y-axis be the common radical axis of a coaxal system of circles. Then any circle of the coaxal system is

x2 + y2 + 2lx + c = 0

(5.1)

where l is a variable and c is a constant.

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System of Circles

5-5

The limiting points of this coaxal system of circles are ( c , 0) and ( − c , 0). The polar of the point ( c , 0) with respect to the circle (5.1) is cx + l( x + c ) + c = 0



(i.e.)  ( l + c )( x + c ) = 0 since l + c ≠ 0, x + c = 0.

This line passes through the other limiting point. For every coaxal system of circles there exists an orthogonal system of circles. Let x-axis be the line of centres and y-axis be the common radical axis. Then the equation to a coaxal system of circles is x2 + y2 + 2lx + c = 0 (5.2) Let us assume that the circle

x2 + y2 + 2gx + 2fy + k = 0

(5.3)

cut every circle of the coaxal system of circles given by (5.2) orthogonally. Then the condition for orthogonality is 2gl = c + k



(5.4)

Let us now consider two circles of the coaxal system for the different values of l, say l1 and l2. The condition (5.4) becomes 2gl1 = c + k, 2gl2 = c + k.

∴ 2(l1 - l2)g = 0.

Since l1 - l2 ≠ 0, g = 0 and so k = −c. Hence, from (5.3) the equation of the circle which cuts every member of the system (5.2) is x2 + y2 + 2fy - c = 0, where f is an arbitrary constant. Therefore, for every coaxal system of circles there exists an orthogonal system of circles given by x2 + y2 + 2fy - c = 0; where f is a variable and c is a constant. For this system of orthogonal circles y-axis is the line of centres and x-axis is the common radical axis.

Note 5.4.1:  Every circle of the orthogonal coaxal system of circles passes through the limiting points ( ± c , 0).

Theorem 5.4.2:  If S = x2 + y2 + 2gx + 2fy + c = 0 and S1 = x2 + y2 + 2g1x + 2f1y + c1 = 0 be any two circles of a coaxal system then any circle of coaxal system can be expressed in the form S + lS1 = 0.

Proof: 

S = x2 + y2 + 2gx + 2fy + c = 0

(5.5)



S1 = x + y + 2g1x + 2f1y + c1 = 0

(5.6)

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5-6

Analytical Geometry

Consider, S + lS1 = 0 (5.7) 2 where l is a variable. In this equation, (1 + l) is the coefficient of x and y2. S + lS1 = 0 in which the Dividing by (1 + l) equation (5.7) becomes 1+ l coefficient of x2 and y2 are unity. S + l1S1 =0 Now consider two different values of l, that is, l1 and l2. Then, 1 + l1 S + l2 S1 and 1 + l = 0 . 2 S + l1S1 S + l2 S1 − =0 The radical axis of these two circles is 1 + l1 1 + l2 (i.e.)  (1 + l2) (S + l1S1) - (1 + l1) (S + l2S1) = 0 (i.e.)  (l1 - l2) (S - S1) = 0. Since l1 - l2 ≠ 0 and therefore S - S1 = 0 which is the common radical axis. Therefore, every member of the coaxal system can be expressed in the form S + lS1 = 0 where l is a variable.

Theorem 5.4.3:  If S = x2 + y2 + 2gx + 2fy + c = 0 is a circle of a coaxal system and L = lx + my + n = 0 is the common radical axis of the system then S + l L = 0 is the equation of a circle of the coaxal system of circles. Proof: 

S = x2 + y2 + 2gx + 2fy + c = 0 (5.8) L = lx + my + n = 0 (5.9) S + l L = 0 (5.10) (i.e.)  x2 + y2 + 2gx + 2fy + c + l (lx + my + n) = 0 Consider two members of the system (5.10) for the different values of l , that is, l1 and l2. Then, S + l1L = 0 and S + l2L = 0 The radical axis of these two circles is (l1 - l2)L = 0. Since l1 - l2 ≠ 0, L = 0 which is the common radical axis. Therefore, S + l L = 0 represents any circle of the coaxal system in which S = 0 is a circle and L = 0 is the common radical axis.

5.5  EXAMPLES (RADICAL AXIS) Example 5.5.1 Find the radical axis of the two circles x2 + y2 + 2x + 4y - 7 = 0 and x2 + y2 - 6x + 2y - 5 = 0 and show that it is at right angles to the line of centres of the two circles.

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System of Circles

5-7

Solution x2 + y2 + 2x + 4y - 7 = 0 x2 + y2 - 6x + 2y - 5 = 0



The radical axis of the circles is S - S1 = 0. (i.e.)  8x + 2y - 2 = 0 (i.e.)  4x + y - 1 = 0 The slope of the radical axis is m1 = -4. The centres of the two circles are (−1, −2) and (3, −1). −2 + 1 1 = . The slope of the line of centres is m2 = −1 − 3 4  1 m1m2 = ( −4)   = −1  4

(5.11) (5.12)

(5.13)

Therefore, the radical axis is perpendicular to the line of centres.

Example 5.5.2 Show that the circle x2 + y2 + 2gx + 2fy + c = 0 will bisect the circumference of the circle x2 + y2 + 2g1x + 2f1y + c1 = 0, if 2g1(g - g1) + 2f1(f - f1) = c - c1.

Solution Let

x2 + y2 + 2gx + 2fy + c = 0 x2 + y2 + 2g1x + 2f1 y + c1 = 0

 

(5.14) (5.15)

The radical axis of these two circles is 2(g - g1)c + 2( f - f1)y + c - c1 = 0, Circle (5.14) bisects the circumference of the circle (5.15). Therefore, radical axis passes through the centre of the second circle. The radical axis of the two given circles be 2(g - g1) (−g1) + 2( f - f1) (−f1) + c - c1 = 0

(i.e.)  2(g - g1) g1 + 2( f - f1) f1 - (c - c1) = 0

Example 5.5.3 Show that the circles x2 + y2 - 4x + 6y + 8 = 0 and x2 + y2 - 10x – 6y + 14 = 0 touch each other and find the coordinates of the point of contact.

Solution

x2 + y2 - 4x + 6y + 8 = 0 x2 + y2 - 10x - 6y + 14 = 0

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(5.16) (5.17)

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5-8

Analytical Geometry

The radical axis of these two circles is 6x + 12y - 6 = 0. (i.e.)  x + 2y - 1 = 0



(5.18)

The centres of the circles are A (2, −3) and B (5, 3). The radii of the circles are r1 = 4 + 9 − 8 = 5 and r2 = 25 + 9 − 14 = 2 5. The perpendicular distance from A(2, −3) on the radical axis x + 2y - 1 = 0 12 − 6 − 11 5 = = 5 = radius of the first circle. is 1+ 4 5 Therefore, radical axis touches the first circle and hence the two circles touch each other. y + 3 −3 − 3 −6 = = =2 The equation of the lines of centres is x −2 2−5 −3 or

2x - 4 = y + 3 ⇒ 2x - y - 7 = 0

(5.19)

Solving (5.18) and (5.19), we get the point of contact. Therefore, the point of contact is (3, −1).

Example 5.5.4 Show that the circles x2 + y2 + 2ax + c = 0 and x2 + y2 + 2by + c = 0 touch if 1 1 1 + 2 = . 2 c a b

Solution The radical axis of the two given circles is 2ax - 2by = 0. The centre of the first circle is (−a, 0). The radius of the first circle is a 2 − c . If the two circles touch each other, then the perpendicular distance from the centre (−a, 0) to the radical axis is equal to the radius of the circle. \

a2 a2 + b2

= a 2 − c ⇒ a 4 = ( a 2 − c)( a 2 + b 2 )



⇒ a4 = a4 - c(a2 + b2) + a2b2



⇒ c(a2 + b2) = a2b2

On dividing by a2b2c, we get

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1 1 1 + 2 = . 2 c a b

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System of Circles

5-9

Example 5.5.5 Find the radical centre of the circles x2 + y2 + 4x + 7 = 0, 2x2 + 2y2 + 3x + 5y + 9 = 0 and x2 + y2 + y = 0.

Solution Let  x2 + y2 + 4x + 7 = 0

(5.20)

3 5 9 x + y + = 0 2 2 2

(5.21)

 x2 + y2 + y = 0

(5.22)



x2 + y2 +

5 5 5 x − y + = 0. 2 2 2 ⇒ x - y + 1 = 0

The radical axis of circles (5.20) and (5.22) is

(5.23)

The radical axis of the circles (5.20) and (5.22) is 4x - y + 7 = 0



(5.24)

Solving (5.23) and (5.24) we get the radical centre as follows: x − y = −1 4 x − y = −7



3 x = −6

⇒ x = −2

−2 - y = −1 y = −2 + 1 = −1



Therefore, the radical centre is (−2, −1).

Example 5.5.6 Prove that if the points of intersection of the circles x2 + y2 + ax + by + c = 0 and x2 + y2 + a1x + b1y + c1 = 0 by the lines Ax + By + C = 0 and A1x + B1y + C1 = 0 are concyclic if

a − a1 A A1

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b − b1 B B1

c − c1 C = 0. C1

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5-10

Analytical Geometry

Solution Let

x2 + y2 + ax + by + c = 0

(5.25)



x2 + y2 + a1x + b1y + c1 = 0

(5.26)



Ax + By + C = 0

(5.27)



A1x + B1y + C1 = 0

(5.28)

Ax + By + C = 0 meets the circle (5.25) at P and Q and A1x + B1y + C1 = 0 meets the circle (5.26) at R and S. Since P, Q, R and S are concyclic, the equation of this circle be

x2 + y2 + 2gx + 2fy + c = 0

(5.29)

The radical axis of the circles (5.25) and (5.26) is

(a - a1) x + (b - b1) y + (c - c1) = 0

(5.30)

The radical axis of circles (5.25) and (5.29) is Ax + By + C = 0



(5.31)

The radical axis of circles (5.26) and (5.29) is A1x + B1y + C1 = 0



(5.32)

Since these three radical axes are concurrent we get from equations (5.30), (5.31) and (5.32),

a − a1 A A1

b − b1 B B1

c − c1 C = 0. C1

Example 5.5.7 Prove that the difference of the square of the tangents to two circles from any point in their plane varies as the distance of the point from their radical axis.

Solution Let P(x1, y1) be any point and the two circles be

x2 + y2 + 2gx + 2fy + c = 0

(5.33)



x2 + y2 + 2g1x + 2f1y + c1 = 0

(5.34)

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System of Circles

5-11

The equation to the radical axis of these two circles be

2(g - g1) x + 2( f - f1) y + c - c1 = 0



PT 2 - PT 12 = (x12 + y12 + 2gx1 + 2fy1 + c) - (x2 + y12 + 2g1x1 + 2f1y1 + c1)



PT 2 - PT 12 = 2(g - g1) x1 + 2( f - f1) y + c - c1

(5.35)

(5.36)

The perpendicular distance of the point from the radical axis is PR =



2( g − g1 ) x1 + 2( f − f1 ) y1 + c − c1 2 ( g − g1 )2 + ( f − f1 )2



(5.37)

From equations (5.36) and (5.37), we get PT 2 - PT 12 ∝ PR.

Example 5.5.8 Prove that for all constants l and m, the circle (x - a) (x - a + l) + (y - b) (y - b + m) = r2 bisects the circumference of the circle (x - a)2 + (y - b)2 = r2.

Solution

(x - a)2 + (y - b)2 + l(x - a) + m (y - b) −r2 = 0

(i.e.)  x2 + y2 - x(l - 2a) + y(m - 2b) + a2 + b2 - la + mb - r2 = 0

(x - a)2 + (y - b)2 = r2



(i.e.)  x2 + y2 - 2ax - 2by + a2 + b2 = 0

(5.38) (5.39)

The radical axis of these two circles is lx + my - la - mb = 0 (5.40) The centre of the second circle is (a, b). Substituting x = a, y = b in (5.40), we get la + mb − la - mb = 0. ∴ (a, b) lies on the radical axis. Therefore, the radical axis bisects the circumference of the second circle.

Example 5.5.9 Prove that the length of common chord of the two circles x2 + y2 + 2lx + c = 0 and x2 + y2 + 2my - c = 0 is 2

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( l 2 − c)( m 2 + c) . l2 + m2

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5-12

Analytical Geometry

Solution The two given circles are C L A (−λ, 0)

B D

x2 + y2 + 2lx + c = 0  x2 + y2 + 2my - c = 0 Centres are A(−l, 0) and B (−m, 0), radii are AC = l 2 − c and BC = m 2 + c . The radical axis is lx - my + c = 0. The perpendicular distance from A on l2 − c CD is AL = . l2 + m2 ∴ CL2 = AC 2 − AL2 = ( l 2 − c) −



( l 2 − c)2 l2 + m2

( l 2 − c)( l 2 + m 2 ) − ( l 2 − c)2 l2 + m2 2 ( l − c)[l 2 + m 2 − l 2 + c] = l2 + m2 2 ( l − c)( m 2 + c) = l2 + m2 =



Therefore, the length of common chord = CD = 2CL = 2

( l 2 − c)( m 2 + c) . l2 + m2

Example 5.5.10 Show that the circle x2 + y2 - 8x - 6y + 21 = 0 is orthogonal to the circle x2 + y2 - 2y - 15 = 0. Find the common chord and the equation of the circle passing through the centres and intersecting points of the circles.

Solution

x2 + y2 - 8x - 6y + 21 = 0 x2 + y2 - 2y - 15 = 0

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System of Circles

5-13

The condition for orthogonality is 2gg1 + 2ff1 = c + c1. (i.e.)  2(-4) (0) + 2(−3) (−1) = 21 - 15



0 + 6 = 6 which is true. Therefore, the two circles cut each other orthogonally. The equation of the common chord is S - S1 = 0.

(i.e.)  -8x - 4y + 36 = 0



(i.e.)  2x + y - 9 = 0

Any circle passing through the intersection of the circles is S + lL = 0.

(i.e.)  x2 + y2 - 8x - 6y + 21 + l(2x + y - 9) = 0.

This passes through the centre (4, 3) of the first circle.

16 + 9 - 32 - 18 + 21 + l(8 + 3 - 9) = 0



⇒ -4 + l(2) = 0



⇒l=2

Therefore, the equation of the required circle is x2 + y2 - 8x - 6y + 21 + 2(2x + y - 9) = 0.

(i.e.)  x2 + y2 - 4x - 4y + 3 = 0

Example 5.5.11 Find the equation to the circle which cuts orthogonally the three circles x2 + y2 + 2x + 17y + 4 = 0, x2 + y2 + 7x + 6y + 11 = 0 and x2 + y2 - x + 22f + 33 = 0.

Solution Let the equation of the circle which cuts orthogonally the three given circles be x2 + y2 + 2gx + 2fy + c = 0. Then the conditions for orthogonality are

2g + 17f = c + 4 7g + 6f = c + 11

(5.41) (5.42)

−g + 22f = c + 3

(5.43)

(5.41) - (5.42) gives

(5.44)

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-5g + 11f = -7

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5-14

Analytical Geometry

(5.42) - (5.43) gives

3g - 5f = 1

(5.44) × 3 gives

(5.45)

−15g + 33f = −21

(5.45) × 5 gives

 

15 g − 25 f = 5 8 f = 16

 \ f = −2 From (5.45), we get 3g + 10 = 1 −9 3 = −3 From (5.41), we get -6 - 34 = c + 4 or c = -44 Therefore, the equation of the circle which cuts orthogonally the three given circles is x2 + y2 - 6x - 4y - 44 = 0. g=



Aliter The radical axis of circles (5.41) and (5.42) is 5x - 11y + 7 = 0



(5.46)

The radical axis of circles (5.41) and (5.43) is

3x - 5y + 1 = 0

(5.41) × 3:

15x - 33y + 21 = 0

(5.47) × 5:

15x - 25y + 5 = 0



- 8y + 16 = 0



∴ y = 2; x = 3

(5.47)

Therefore, radical centre is (3, 2). If R is the length of the tangent from points (3, 2) to the first circle then  R2 = 9 + 4 + 6 + 34 + 4 = 57. Therefore, the equation of the required circle is (x - 3)2 + (y - 2)2 = 57.

(i.e.)  x2 + y2 - 6x - 4y - 44 = 0

Example 5.5.12 Find the equation of the circle which passes through the origin, has its centre on the line x + y = 4 and cuts orthogonally the circle x2 + y2 - 4x + 2y + 4 = 0.

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System of Circles

5-15

Solution Let the equation of the required circle passing through the origin be

x2 + y2 + 2gx + 2fy = 0

(5.48)

This circle cuts orthogonally the circle

x2 + y2 - 4x + 2y + 4 = 0 (i.e.)   -4g + 2f = 0 + 4



(5.49) (5.50)

The centre of the circle (5.48) lies on x + y = 4.

\ −g - f = 4



(5.50)



(5.51)

Adding, we get

(5.51)

-4g + 2f  = 4 × 2 −2g − 2 f = 8 6 g = 12 ⇒ g = −2

\ f = −2 Therefore, the equation of the required circle is x2 + y2 - 4x - 4y = 0.

Example 5.5.13 If the equation of the circles with radii r and R are S = 0 and S1 = 0, respecS S tively then show that the circles + 1 = 0 will intersect orthogonally. r R

Solution Without loss of generality, we can assume the line of centres of the two circles as x-axis and the distance between the centres as 2a. Then the centres of the two circles are (a, 0) and (−a, 0). The equation of the two circles are S = (x - a)2 + y2 - r2 = 0 and S1 = (x + a)2 + y2 - R2 = 0. S S Consider ± 1 = 0 r R ∴RS ± rS1 = 0 Clearly, the coefficients of the R and r in these equations are the same and so they represent circles. Consider RS + rS1 = 0

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5-16

Analytical Geometry

(i.e.)

R[ x 2 + y 2 − 2ax + a 2 − r 2 ] + r[ x 2 + y 2 + 2ax + a 2 − R2 ] = 0

or

( R + r )( x 2 + y 2 ) − 2a( R − r ) x + a 2 ( R + r ) − Rr ( R + r ) = 0

(5.52)

Also, RS - rS = 0 has the equation

( R − r )( x 2 + y 2 ) − 2a( R + r ) x + ( R − r )a 2 − Rr ( R − r ) = 0

(5.53)

 R − r Equations (5.52) and (5.53) can be written as x 2 + y 2 − 2a  x + a2 − r  R + r   R + r R = 0 and x 2 + y 2 − 2a  x + ( R + r )a 2 + Rr = 0 .  R − r  The condition for orthogonality is 2gg1 + 2ff1 = c + c1.

(i.e.)  2a 2

( R2 − r 2 ) + 0 = a 2 − rR + a 2 + rR ( R2 − r 2 ) (i.e.)  2a 2 = 2a 2 which is true.

Therefore, the circles

S S1 ± = 0 are orthogonal. r R

Exercises (Radical Axis) 1.  2.  3. 

Find the radical axis of the circles x2 + y2 + 2x + 4y = 0 and 2x2 + 2y2 - 7x - 8y + 1 = 0. Ans.: 11x + 16y - 1 = 0 Find the radical axis of the circles x2 + y2 - 4x - 2y - 11 = 0 and x2 + y2 - 2x - 6y + 1 = 0 and show that the radical axis is perpendicular to the line of centres. Ans.: x - 2y + 5 = 0 Show that the circles x2 + y2 - 6x - 9y + 13 = 0 and x2 + y2 - 2x - 16y = 0 touch each other. Find the coordinates of point of contact. Ans.: (5, 1)

4.

Find the equation of the common chord of the circles x2 + y2 + 2ax + 2by + c = 0 and x2 + y2 + 2bx + 2ay + c = 0 and also show that the circles touch if (a + b)2 = 2c.

5.

Show that the circles x2 + y2 + 2x - 8y + 8 = 0 and x2 + y2 + 10x - 2y + 22 = 0 touch each other and find the point of contact.  −17 11 ,  Ans.:  5 5



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5-17

System of Circles

6.  7.



Find the equation of the circle passing through the intersection of the circles   x2 + y2 = 6 and x2 + y2 - 6x + 8 = 0 and also through the point (1, 1). Ans.: x2 + y2 - x - y = 0 Find the equation of the circle passing through the point of intersection of the circles x2 + y2 - 6x + 2y + 4 = 0 and x2 + y2 + 2x - 4y - 6 = 0 and whose radius is 3/2. Ans.: 5x2 + 5y2 - 18x + y + 5 = 0

8.

If the circles x2 + y2 + 2gx + 2fy = 0 and x2 + y2 + 2g1x + 2f1y = 0 touch each other then show that fg1 = f1g.

9. 

Find the radical centre of the circles x2 + y2 + aix + bi y + c = 0, i = 1, 2, 3. Ans.: (0, 0)

10.

Find the radical centre of the circles x2 + y2 - x + 3y - 3 = 0, x2 + y2 - 2x + 2y + 2 = 0 and x2 + y2 + 2x + 2y - 9 = 0. Ans.: (2, 1)

 11.

 12. 

The radical centre of three circles is at the origin. The equation of two of the circles are x2 + y2 = 1 and x2 + y2 + 4x + 4y - 1 = 0. Find the general form of the third circle. If it passes through (1, 1) and (−2, 1) then find its equation. Ans.: x2 + y2 + x - 2y - 1 = 0 Find the radical centre of the circles x2 + y2 + x + 2y + 3 = 0, x2 + y2 + 4x + 7 = 0 and 2x2 + 2y2 + 3x + 5y + 9 = 0. Ans.: (−2, −1)

13.

Find the equation of the circle whose radius is 3 and which touches the circle x2 + y2 - 4x - 6y + 2 = 0 internally at the point (−1, −1).

14.

Show that the radical centres of three circles described on the sides of a triangle as diameter is the orthocentre of the triangle.

15.

Find the equation of the circle which cuts orthogonally the three circles x2 + y2 + y = 0, x2 + 4y2 + 4x + 7 = 0, 21x2 + y2 + 3x + 5y + 9 = 0. Ans.: x2 + y2 + 4x + 2y + 1 = 0

 16.

A and B are two fixed points and P moves so that PA = n⋅PB. Show that the locus of P is a circle and that for different values of n, all the circles have the same radical axis.

17.

Find the equation of circle whose radius is 5 and which touches the circle x2 + y2 - 2x - 4y - 20 = 0 at the point (5, 5).

18.

Prove that the length of the common chord of the two circles whose equations are (x - a)2 + (y - b)2 = r2 and (x - a)2 + (y - b)2 = c2 is

19.

ac 2 − 2( a − b)2 .

Find the equation to two equal circles with centres (2, 3) and (5, 6) which cuts each other orthogonally.

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5-18

Analytical Geometry

20.

If three circles with centres A, B and C cut each other orthogonally in pairs then prove that the polar of A with respect to the circle centre B passes through C.

21.

Find the locus of centres of all the circles which touch the line x = 2a and cut the circle x2 + y2 = a2 orthogonally.

22.

A, B are the points (a, 0) and (−a, 0). Show that if a variable circle S is orthogonal to the circle on AB as diameter, the polar of (a, 0) with respect to the circle S passes through the fixed point (−a, 0).

23.

If a circle passes through the point (a, b) and cuts the circle x2 + y2 = k2 orthogonally then prove that the locus of its centres is 2ax + 2by - (a2 + b2 + k2) = 0.

24.

Show that the circles x2 + y2 + 10x + 6y + 14 = 0 and x2 + y2 - 4x + 6y + 8 = 0 touch each other at the point (3, −1).

25.

Show that the circles x2 + y2 + 2ax + 4ay - 3a2 = 0 and x2 + y2 - 8ax - 6ay + 7a2 = 0 touch each other at the point (a, 0).

26.

The equation of three circles are x2 + y2 = 1, x2 + y2 + 8x + 15 = 0 and x2 + y2 + 10y + 24 = 0. Determine the coordinate of the point such that the tangents drawn from it to the three circles are equal in length.

27.

If P and Q be a pair of conjugate points with respect to a circle S = 0 then prove that the circle on PQ as diameter cuts the circle S = 0 orthogonally.

28.

Find the equation of the circle whose diameter is the common chord of the circles x2 + y2 + 2x + 3y + 1 = 0 and x2 + y2 + 4x + 3y + 2 = 0.

5.6  Examples (LIMITING POINTS) Example 5.6.1 If A, B and C are the centres of three coaxal circles and t1, t2 and t3 are the lengths of tangents to them from any point then prove that BC ⋅ t12 + CA ⋅ t22 + AB ⋅ t32 = 0.

Solution Let the three circles of coaxal system be

S1 = x2 + y2 + 2g1x + c = 0



S2 = x2 + y2 + 2g2x + c = 0



S3 = x2 + y2 + 2g3x + c = 0

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System of Circles

5-19

The centres are A(−g1, 0), B(−g2, 0) and C(−g3, 0) and BC = g3 - g2, CA = g1 - g3, AB = g2 - g1



t12 = x12 + y12 + 2 g1 x1 + c t22 = x12 + y12 + 2 g 2 x1 + c



t32 = x12 + y12 + 2 g3 x1 + c

Then,

ΣBC ⋅ t12 = − Σ ⋅ ( g 2 − g3 )( x12 + y12 + 2 g1 x1 + c) = −( x12 + y12 + c)Σ ( g 2 − g3 ) + 2 x1Σg1 ( g 2 − g3 )



=0 since Σ (g2 - g3) = 0 and Σg1(g2 - g3) = 0.

Example 5.6.2 Find the equations of the circles which pass through the points of intersection of x2 + y2 - 2x + 1 = 0 and x2 + y2 - 5x - 6y - 4 = 0 and which touch the line 2x - y + 3 = 0.

Solution

x2 + y2 - 2x + 1 = 0

(5.54)



x2 + y2 - 5x - 6y + 4 = 0

(5.55)

The radical axis of these two circles is 3x + 6y - 3 = 0 or x + 2y - 1 = 0



(5.56)

The equation of any circle passing through the intersection of these two circles is x2 + y2 - 2x + 1 + l(x + 2y - 1) = 0. 2  l  l  The centre of this circle is 1 − , − l  and radius = 1 −  + l 2 + l − 1    2 2 The circle (5.56) touches the line 2x - y + 3 = 0.



∴

 l 2 1 −  + l + 3  2 4 +1

2

 l = 1 −  + l 2 + l − 1  2 2



 l ∴ 5 = 1 −  + l 2 + l − 1  2

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5-20

Analytical Geometry

= 1+



⇒5=

⇒



l2 − l + l 2 + l −1 4

l2 + l2 4

5l 2 =5 4 ⇒ l = ±2

The equation of the circles are x2 + y2 - 2x + 1 ± 2(x + 2y - 1) = 0. (i.e.)  x2 + y2 - 2x + 1 + 2x + 4y - 2 = 0 and x2 + y2 - 2x + 1 - 2x - 4y + 2 = 0 (i.e.)  x2 + y2 + 4y - 1 = 0 and x2 + y2 - 4x + 4y + 3 = 0

Example 5.6.3 Find the equation of the circle which passes through the intersection of the circles x2 + y2 = 4 and x2 + y2 - 2x - 4y + 4 = 0 and has a radius 2 2.

Solution

x2 + y2 - 4 = 0 x2 + y2 - 2x - 4y + 4 = 0

(5.57) (5.58)

The radical axis of these two circles is 2x + 4y - 8 = 0. Any circle passing through the intersection of these two circles is

S + lL = 0 ⇒ x2 + y2 - 4 + l(2x + 4y - 8) = 0

Centre is (−l, −2l).

radius = l 2 + 4 l 2 + 4 + 8l = 2 2 ⇒ 5l2 + 8l + 4 = 8 ⇒ 5l2 + 8l - 4 = 0 ⇒ 5l2 + 10l - 2l - 4 = 0 ⇒ 5l (l + 2) - 2(l + 2) = 0 ⇒ (l + 2) (5l - 2) = 0

2 5 Therefore, the required circles are x2 + y2 - 4 - 2(2x + 4y - 8) = 0 and 2 x 2 + y 2 − 4 + (2 x + 4 y − 8) = 0 . 5 (i.e.)  x2 + y2 - 4x - 8y + 12 = 0 and 5x2 + 5y2 + 4x + 8y - 36 = 0

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⇒ l = −2,

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System of Circles

5-21

Example 5.6.4 Find the equation of the circle whose diameter is the common chord of the circles x2 + y2 + 2x + 3y + 1 = 0 and x2 + y2 + 4x + 3y + 2 = 0.

Solution x2 + y2 + 2x + 3y + 1 = 0 x2 + y2 + 4x + 3y + 2 = 0



The radical axis of these two circles is 2x + 1 = 0. Any circle of the system is x2 + y2 + 2x + 3y + 1 + l(2x + 1) = 0. −3   Centre is  −1 − l,  . 2 Since the radical axis is a diameter, centre lies on the radical axis.

∴ 2(−1 - l) + 1 = 0

−2 - 2l + 1 = 0

⇒ 2l = −1 l=

or

−1 2

Hence, the equation of the required circle is 1 1 − (2 x + 1) = 0. 2

⇒ 2x2 + 2y2 + 4x + 6y + 2 - 2x - 1 = 0



⇒ 2x2 + 2y2 + 2x + 6y + 1 = 0

x2 + y2 + 2x + 3 y +

Example 5.6.5 Find the equation of the circle which touches x-axis and is coaxal with the circles x2 + y2 + 12x + 8y - 33 = 0 and x2 + y2 = 5.

Solution

x2 + y2 + 12x + 8y - 33 = 0



x2 + y2 - 5 = 0

The radical axis of these two circles is

12x + 8y - 28 = 0 (i.e.)  6x + 4y - 14 = 0

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(5.59)

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5-22

Analytical Geometry

Any circle of the coaxal system is x2 + y2 - 5 + l (6x + 4y - 14) = 0. Centre is (−3l, −2l). Radius = 9l 2 + 4 l 2 + 5 + 14 l

= 13l 2 + 14 l + 5

The circles (5.69) touches x-axis (i.e.) y = 0. Radius = ±

2l 1

= ± 2l ⇒ 13l 2 + 4 l + 5 = 4 l 2



 9l 2  14 l  5  0



⇒ (l + 1) (9l + 5) = 0  ⇒ l = −1  or 

Therefore, the two circles of the system touching x-axis are

−5 9

x2 + y2 - 5 - 1 (6x + 4y - 14) = 0 ⇒ x2 + y2 - 6x - 4y + 9 = 0

and 9(x2 + y2 - 5) - 5(6x + 4y - 14) = 0 ⇒ 9x2 + 9y2 - 36x - 20y + 25 = 0.

Example 5.6.6 The line 2x + 3y = 1 cuts the circle x2 + y2 = 4 in A and B. Show that the equation of the circle on AB as diameter is 13 (x2 + y2) - 4x - 6y - 50 = 0.

Solution Let x2 + y2 - 4 = 0 (5.60) 2x + 3y - 1 = 0 (5.61) Any circle passing through the intersection of the circle and the line is

x2 + y2 + l(2x + 3y - 1) = 0

(5.62)

3l   9l +l. Centre is  − l , −  and radius = l 2 + 2 4 If AB is a diameter of the circle (5.72), their centre should lie on AB. 2



 −3l  −2l + 3  −1 = 0  2 

−2 13 Therefore, the equation of the circle on AB as diameter is 13(x2 + y2 - 4) 2(2x + 3y - 1) = 0. ∴ 13(x2 + y2) - 4x - 6y + 50 = 0

⇒ −4 l − 9l − 2 = 0 ⇒ l =

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System of Circles

5-23

Example 5.6.7 A point moves so that the ratio of the length of tangents to the circles x2 + y2 + 4x + 3 = 0 and x2 + y2 - 6x + 5 = 0 is 2:3. Show that the locus of the point is a circle coaxal with the given circles.

Solution The lengths of tangents from a point P(x1, y1) to the two circles are PT12 = x12 + y12 + 4 x1 + 3 ; PT22 = x12 + y12 − 6 x1 + 5. Given that,

PT12 4 4 = ⇒ PT12 = PT22 2 9 9 PT2 4 2 2 2 2 ⇒ x1 + y1 + 4 x1 + 3 = ( x1 + y1 − 6 x1 + 5) 9 4 2 2 The locus of (x1, y1) is x + y + 4 x2 + 3 − ( x 2 + y 2 − 6 x2 + 6) = 0 9 This is of the form S1 + lS2 = 0 Hence the locus of circle is a circle coaxal with the two given circles.

Example 5.6.8 Find the limiting points of the coaxal system determined by the circle x2 + y2 + 2x + 4y + 7 = 0 and x2 + y2 + 4x + 2y + 5 = 0.

Solution Given that, x2 + y2 + 2x + 4y + 7 = 0 x2 + y2 + 4x + 2y + 5 = 0 The radical axis of these two circles is 2x - 2y - 2 = 0. Any circle of the coaxal system is x2 + y2 + 2x + 4y + 7 + l (2x - 2y - 2) = 0. Centre is (−1 −l, −2 + l). Radius is (1 + l )2 + (2 − l )2 + 2l − 7 = 2l 2 − 2 . Limiting points are the centres of circles of radii zero. Therefore, limiting points are (−2, −1) and (0, −3).

Example 5.6.9 The point (2, 1) is a limiting point of a system of coaxal circles of which x2 + y2 - 6x - 4y - 3 = 0 is a member. Find the equation to the radial axis and the coordinates of the other limiting point.

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5-24

Analytical Geometry

Solution Given that x2 + y2 - 6x - 4y - 3 = 0



Since (2, 1) is a limit point, the point circle corresponding to the coaxal system is (x - 2)2 + (y - 1)2 = 0 ⇒ x + y2 - 4x - 2y + 5 = 0



2

The radical axis of the system is S - S1 = 0  or  2x + 2y + 8 = 0 ⇒ x+y+4=0



Any circle of the coaxal system is S + lL = 0. x2 + y2 - 6x - 4y - 3 + l(2x + 2y + 8) = 0



Centre is (3 - l, 2 - l). Radius = (3 − l )2 + (2 − l )2 + (3 − 8l ) For point circles, radius = 0.

⇒ (3 - l)2 + (2 - l)2 + 3 - 8l = 0



⇒ 2l2 - 18l + 16 = 0



⇒ l2 - 9l + 8 = 0

\ l = 1, 8 Therefore, the limiting points are the centres of point circle of the coaxal system, that is, (2, 1) and (-5, -6).

Example 5.6.10 Find the equation of the circle which passes though the origin and belongs to the coaxal system of which limiting points are (1, 2) and (4, 3).

Solution Since (1, 2) and (4, 3) are limiting points of two circles of the coaxal system and (x - 1)2 + (y - 2)2 = 0 and (x - 4)2 + (y - 3)2 = 0.

(i.e.)  x2 + y2 - 2x - 4y + 5 = 0



 x2 + y2 - 8x - 6y + 25 = 0

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System of Circles

5-25

Radical axis is 6x + 2y - 20 = 0. Any circle of the system is x2 + y2 - 2x - 4y + 5 + l (6x + 2y - 20) = 0. This passes through the origin. ∴ 5 − 20 l = 0 ⇒ l =



1 4

Hence, the equation of the system is 4(x2 + y2 - 2x - 4y + 5) + 6x + 2y - 20 = 0.

⇒ 4x2 + 4y2 - 2x + 4y = 0



⇒ 2x2 + 2y2 - x - 7y = 0

Example 5.6.11 A point P moves so that its distances from two fixed points are in a constant ratio l. Prove that the locus of P is a circle. If l varies then show that P generates a system of coaxal circles of which the fixed points are the limiting points.

Solution Let P(x1, y1) be a moving point and A(c, 0) and B(0, −c) be the two fixed points. Here, we have chosen the fixed points on the x-axis such that P is its midpoint. Given that

PA = l ⇒ PA2 = l 2 PB 2 ⇒ ( x − a)2 + y 2 − l 2 [( x + a)2 + y 2 ] = 0 PB

This equation is of the form S + lS′ = 0 which is the equation to a coaxal system of circles. Therefore, for different values of l, P generates a coaxal system of circles of which (x - a)2 + y2 = 0 and (x + a)2 + y2 = 0 are members. These equations are the equation of point circles whose centres are (a, 0), (−a, 0) which is the fixed points.

Example 5.6.12 x 2 + y 2 + 2 gx + c + c k l(x 2 + y 2 + 2 fy + k ) = 0 subtends a right angle at the origin if 2 + 2 = 2 . g f

Prove that the limiting point of the system

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5-26

Analytical Geometry

Solution The two members of the system are x2 + y2 + 2gx + c = 0 and x2 + y2 + 2fy + k = 0. Radical axis is 2 gx − 2 fy + c − k = 0 . Any circle of the system is x 2 + y 2 + 2 gx + c + l(2 gx − 2 fy + c − k ) = 0 . Centre is (−g - gl, fl). Radius = ( g + g l )2 + f 2 l 2 − c − cl + k l For point circle, radius = 0. ( g + g l ) 2 + l 2 f 2 − c − cl + k l = 0



g 2 ( l 2 + 2l + 1) + f 2 l 2 − c − cl + k l = 0

 l 2 ( g 2 + f 2 ) + l (2 g 2 − c + k ) + g 2 − c = 0 Considering the two values of l as l1, l2, centres are A(−g(1 + l1), fl1) and B(−g(1 + l2), fl2) Since OAB is right angled at O, OA is perpendicular to OB.  ∴

⇒ f 2 l1l2 + g 2 (1 + l1 + l2 + l1l2 ) = 0



f 2 l1l2 = −1 g 2 (1 + l1 )(1 + l2 )

 (2 g 2 − c + k ) g 2 − c   g2 − c  g + + 2 ⇒ f2 2 =0 1 − g2 + f 2 g + f2  g + f 2   ⇒ f 2 ( g 2 − c) + g 2 ( g 2 + f 2 − 2 g 2 + c − k + g 2 − c) = 0 ⇒ f 2 ( g 2 − c) + g 2 ( f 2 − k ) = 0 ⇒ 2 f 2 g 2 = cf 2 + kg 2 c k ⇒ 2 + 2 =2 g f



Exercises 1.  2.

Find the equation of the circle passing through the intersection of x2 + y2 6 = 0 and x 2 + y 2 + 4 y − 1 = 0 through the point (−1, 1). Ans.: 9x2 + 9y2 + 16y - 34 = 0 Show that the circles x 2 + y 2 = 480 and x 2 + y 2 − 10 x − 24 y + 120 = 0 touch each other and find the equation, if a third circle which touches the circles at their point of intersection and the x-axis   x 2 + y 2 − 200 x − 400 y + 10000 = 0.



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Ans.: 5x2 + 5y2 - 40x + 96y + 30 = 0

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5-27

System of Circles

3.

Find the equation of the circle whose centre lies on the line x + y - 11 = 0 and which passes through the intersection of the circle x2 + y2 - 3x + 2y - 4 = 0 with the line 2x + 5y - 2 = 0.

4.

Find the length of the common chord of the circles x2 + y2 + 4x - 22y = 0 and x2 + y2 - 10x + 5y = 0. Ans.: 40/7

 5.

Find the coordinates of the limiting points of the coaxal circles determined by the two circles x 2 + y 2 − 4 x − 6 y − 3 = 0 and x 2 + y 2 − 24 x − 26 y + 277 = 0. Ans.: (1, 2),(3, 1)

 6.

Find the coordinates of the limiting points of the coaxal system of circles of which two members are x 2 + y 2 + 2 x − 6 y = 0 and 2 x 2 + 2 y 2 − 10 y + 5 = 0 .

 7.

Ans.: (1, 2),(3, 1) Find the coaxal system of circles if one of whose members is x 2 + y 2 + 2 x − 6 y = 0 and a limiting point is (1, −2). Ans.: x 2 + y 2 + 2 x + 3 y − 7 − l( 4 x − y − 12) = 0

 8.  9.

Find the limiting point of the coaxal system determined by the circles x 2 + y 2 − 6 x − 6 y + 4 = 0 and x 2 + y 2 − 2 x − 4 y + 3 = 0 .

2 8 Ans.: ( −1, 1),  −   5 5 Find the equation of the coaxal system of circles one of whose members is x 2 + y 2 − 4 x − 2 y − 5 = 0 and the limiting point is (1, 2). Ans.: x 2 + y 2 − 2 x − 4 y + 5 + l( x − y − 5) = 0

 10.

If origin is a limiting point of a system of coaxal circles of which x 2 + y 2 + 2 gx + 2 fy + c = 0 is a member then show that the other limiting  − gc − fc  . points is  2 , 2 2  g + f f + g 2 

11.

Show that the equation of the coaxal system whose limiting points are (0, 0) and (a, b) is x 2 + y 2 + k (2ax − 2by − a 2 − b 2 ) = 0.

12.

The origin is a limiting point of a system of coaxal circles of which x 2 + y 2 + 2 gx + 2 fy + c = 0 is a member. Show that the equation of circles of the orthogonal system is ( x 2 + y 2 )( g + l f ) + c( x − ly ) = 0 for different values of x.

13.

Show that the circles x 2 + y 2 + 2ax + 2by + 2l( ax − by ) = 0 where l is a parameter from a coaxal system and also show that the equation of the common radical axis and the equation of circles which are orthogonal to this x y  system are ax − by = 0, 2( x 2 + y 2 ) + c  + − 2 = 0 . a b 

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5-28

Analytical Geometry

14.

A point P moves such that the length of tangents to the circles x 2 + y 2 − 2 x − 4 y + 5 = 0 and x 2 + y 2 + 4 x + 6 y − 7 = 0 are in the ratio 3:4. Show that the locus is a circle.

15.

Show that the limiting points of the circle x2 + y2 = a2 and an equal circle with centre on the line lx + my + n = 0 be on the line ( x 2 + y 2 )(lx + my + n) + a 2 (ln + mn) = 0 .

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Chapter

6

Parabola

6.1  INTRODUCTION If a point moves in a plane such that its distance from a fixed point bears a constant ratio to its perpendicular distance from a fixed straight line then the path described by the moving point is called a conic. In other words, if S is a fixed point, l is a fixed straight line and P is a moving point and PM SP = a constant, then is the perpendicular distance from P on l, such that PM the ­locus of P is called a conic. This constant is called the eccentricity of the conic and is denoted by e. M

P

S X

If e = 1, the conic is called a parabola. If e < 1, the conic is called an ellipse. If e > 1, the conic is called a hyperbola. The fixed point S is called the focus of the conic. The fixed straight line SP = e is called the focusis called the directrix of the conic. The property PM directrix property of the conic.

6.2  GENERAL EQUATION OF A CONIC We can show that the equation of a conic is a second degree equation in x and y. This is derived from the focus-directrix property of a conic. Let S(x1, y1) be the focus and P(x, y) be any point on the conic and lx + my + n = 0 be the equation of the directrix. The focus-directrix property of the conic SP = e (i.e.) SP2 = e2 PM2 states PM

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6-2

Analytical Geometry

(i.e.)

 lx + my + n  ( x − x1 ) + ( y − y1 ) = e    l 2 + m2  2

2

2

2

This equation can be expressed in the form ax2 + 2hxy + by2 + 2gx + 2fy + c = 0 which is a second degree equation in x and y.

6.3  EQUATION OF A PARABOLA Let S be the focus and the line l be the directrix. We have to find the locus of a point P such that its distance from the focus S is equal to its distance from the fixed line l. SP (i.e.)  = 1 where PM is perpendicular to the directrix. PM Draw SX perpendicular to the directrix and bisect SX. Let A be the point of bisection and SA = AX = a. Then the point A is a point on the parabola since SA = 1. Take AS as the x-axis and AY perpendicular to AS as the y-axis. Then AX the coordinate of S are (a, 0). Let (x, y) be the coordinates of the point P. Draw PN perpendicular to the x-axis. Y M MÄ

X

l

P (x, y)

L A

S(a, 0)

N

x

LÄ

PM = NX = NA + AX = x + a



SP =1 PM

Since

SP2 = PM2



(i.e.)  (x - a)2 + y2 = (x + a)2 y2 = (x + a)2 - (x − a)2 or y2 = 4ax. This, being the locus of the point P, is the equation of the parabola. This equation is the simplest possible equation to a parabola and is called the standard equation of the parabola.

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Parabola

6-3

Note 6.3.1: 1. 2. 3. 4. 5.

The line AS(x-axis) is called the axis of the parabola. The point A is called the vertex of the parabola. AY(y-axis) is called the tangent at the vertex. The perpendicular through the focus is called the latus rectum. The double ordinate through the focus is called the length of the latus rectum. 6. The equation of the directrix is x + a = 0. 7. The equation of the latus rectum is x - a = 0.

6.4  LENGTH OF LATUS RECTUM To find the length of the latus rectum,draw LM′ perpendicular to the directrix. SL =1 Then LM ′ \SL = LM′ = SX = 2a

\LL′ = 2SX = 4a

6.4.1  Tracing of the curve y2 = 4ax 1. If x < 0, y is imaginary. Therefore, the curve does not pass through the left side of y-axis. 2. When y = 0, we get x = 0. Therefore, the curve meets the y-axis at only one point, that is, (0, 0). 3. When x = 0, y2 = 0, that is, y = 0. Hence the y-axis meets the curve at two coincident points (0, 0). Hence the y-axis is a tangent to the curve at (0, 0). 4. If (x, y) is a point on the parabola y2 = 4ax, (x, -y) is also a point. Therefore, the curve is symmetrical about the x-axis. 5. As x increases indefinitely, the values of y also increases indefinitely. Therefore the points of the curve lying on the opposite sides of x-axis extend to infinity towards the positive side of x-axis.

6.5  Different forms of parabola 1.  If the focus is taken at the point (-a, 0) with the vertex at the origin and its axis as x-axis then its equation is y2 = -4ax.

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6-4

Analytical Geometry

y y 2 = − 4ax S A

(−a, 0)

x

2.  If the axis of the parabola is the y-axis, vertex at the origin and the focus at (0, a), the equation of the parabola is x2 = 4ay. y

x 2 = 4ay

S (0, a)

A

x

3.  If the focus is at (0, -a), vertex (0, 0) and axis as y-axis, then the equation of the parabola is x2 = -4ay. y

A x

S (0, −a)

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x 2 = − 4ay

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6-5

Parabola

ILLUSTRATIVE EXAMPLES based on focus directrix property Example 6.1 Find the equation of the parabola with the following foci and directrices:   (i)  (1, 2): x + y - 2 = 0 (ii)  (1, -1): x - y = 0 (iii)  (0, 0): x - 2y + 2 = 0

Solution   (i) Let P(x, y) be any point on the parabola. Draw PM perpendicular to the directrix. Then from the definition of the parabola, SP = 1 PM M

P (x, y)

S (1, 2) X





\ SP2 = (x - 1)2 + (y - 2)2 PM = perpendicular distance from (x, y) on x + y - 2 = 0 ( x + y − 2) =± 2 2  x + y − 2 ∴ ( x − 1)2 +(y − 2)2 =    2  2 2 2 2 \ 2(x - 2x + 1) + 2(y - 4y + 4) = x + y + 4 + 2xy - 4x - 4y (i.e.)  x2 + y2 - 2xy - 4y + 6 = 0 This is the equation of the required parabola.

(ii)  The point S is (1, −1). Directrix is x - y = 0 SP2 = (x - 1)2 + (y + 1)2 = x2 + y2 - 2x + 2y + 2 ( x − y) PM = ± 2 From any point on the parabola, SP = 1 PM \ SP2 = PM2 ⇒ 2(x2 + y2 - 2x + 2y + 2) = x2 + y2 - 2xy (i.e.)  x2 + y2 + 2xy - 4x + 4y + 4 = 0

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6-6

Analytical Geometry

  (iii)  S is (0, 0). Directrix is x - 2y + 2 = 0 SP2 = (x - 0)2 + (y - 0)2 = x2 + y2



PM = ±



( x − 2 y + 2) 1+ 4

=±

( x − 2 y + 2) 5

For any point P on the parabola, SP = 1 PM \SP2 = PM2  x − 2 y + 2 x2 + y2 =    5 



2

(i.e.)  5(x2 + y2) = x2 + 4y2 + 4 - 4xy + 4x - 8y (i.e.)  4x2 + y2 + 4xy - 4x + 8y - 4 = 0

Example 6.2 Find the foci, latus rectum, vertices and directrices of the following parabolas:   (i)  y2 + 4x - 2y + 3 = 0 (ii)  y2 - 4x + 2y - 3 = 0 (iii)  y2 - 8x - 9 = 0

Solution y

S

M (1/2, 1)

(−3/2, 1)

(−1/2, 1)

0

(i)  y2 + 4x - 2y + 3 = 0 y2 - 2y = -4x - 3 y2 - 2y + 1 = -4x - 3 + 1

x

( )

⇒ ( y − 1)2 = −4 x + 1 2

( )

Take x + 1 = X , y - 1 = Y. Shifting the origin to the point −1 , 1 the 2 2 equation of the parabola becomes Y 2 = -4X.

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Parabola

6-7

( )

( ) ( )

\Vertex is −1 , 1 , latus rectum is 4, focus is −3 , 1 and foot of the 2 2 1 directrix is , 1 . The equation of the directrix is x = 1 or 2x - 1 = 0. 2 2 (ii) y2 - 4x + 2y - 3 = 0 y2 + 2y = 4x + 3 ⇒ y2 + 2y + 1 = 4x + 3 + 1 ⇒ (y + 1)2 = 4(x + 1) y 0 x (−1, −1) S (0, −1)

(−2, −1)

Shifting the origin to the point (-1, -1) by taking x + 1 = X and y + 1 = Y the equation of the parabola becomes Y2 = 4X. \Vertex is (-1, -1), latus rectum = 4, focus is (0, -1) and foot of the directrix is (-2, -1). \The equation of the directrix is x + 2 = 0.

y2 − 8x − 9 = 0 ⇒ y2 = 8x + 9

( )

Shift the origin to the point −9 , 0 and take x + 9 = X and y = Y. 8 8 \ The equation of the parabola becomes Y2 = 8X. Vertex is −9 , 0 , 8 latus rectum = 8 andfocus is −9 + 2, 0 (i.e.) 7 , 0 . 8 8 9 The equation of the directrix is x + + 2 = 0. (i.e.) 8x + 25 = 0. 8

(

y

M

X (−25/8, 0)

M06_ANALYTICALGEOMETRY_ISBN_CH06.indd 7

( )

)

( )

0

S

x

(−9/8, 0)

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6-8

Analytical Geometry

Exercises 1.

Find the equation of the parabola whose focus is (2, 1) and directrix is 2x + y + 1 = 0. Ans.: x2 - 4xy + 4y2 - 24x - 12y + 24 = 0

2.

Find the equation of the parabola whose focus is (3, -4) and whose directrix is x - y + 5 = 0. Ans.: x2 + 2xy + y2 - 16x - 26y + 25 = 0

3.

Find the coordinates of the vertex, focus and the equation of the directrix of the parabola 3y2 = 16x. Find also the length of the latus rectum. Ans.: (0, 0);

 4.

Find the coordinates of the vertex and focus of the parabola 2y2 + 3y + 4x = 2. Find also the length of the latus rectum.

5.

( 43 , 0); 163 , 3x + 4 y = 0 ( 3) (

)2

Ans.: −1, 4 ; −1, 23 ; 3

24

A point moves in such a way that the distance from the point (2, 3) is equal to the distance from the line 4x + 3y = 5. Find the equation of its path. What is the name of this curve? Ans.: 25[(x - 2)2 + (y - 3)2] - (4x + 3y - 5)2

6.6  Condition for tangency Find the condition for the straight line y = mx + c to be a tangent to the parabola y2 = 4ax and find the point of contact.

Solution The equation of the parabola is The equation of the line is

y2 = 4ax

(6.1)

y = mx + c

(6.2)

Solving equations (6.1) and (6.2), we get their points of intersection. The x-coordinates of the points of intersection are given by

(mx + c)2 = 4ax ⇒ m2 x2 + 2(mc − 2a) x + c2 = 0

(6.3)

If y = mx + c is a tangent to the parabola, then the roots of this equation are equal. The condition for this is the discriminant is equal to zero.

\ 4(mc- 2a)2 = 4m2c2



⇒ m2 c 2 + 4 a 2 − 4 mca = m2 c 2 ⇒ c = a /m

Hence, the condition for y = mx + c to be a tangent to the parabola y2 = 4ax is c = a /m.

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6-9

Parabola

Substituting c = a /m in equation (6.3), we get

(

2 m2 x 2 − 2ax + a 2 = 0 ⇒ mx − a m m

) =0 2

Therefore, the point of contact is  a2 , 2a  . m m

Note 6.6.1: Any tangent to the parabola is y = mx + a . m

6.7  Number of tangents Show that two tangents can always be drawn from a point to a parabola.

Solution Let the equation to the parabola be y2 = 4ax. Let (x1, y1) be the given point. Any tangent to the parabola is y = mx + a . If this tangent passes through m (x1, y1), then y1 = mx1 + a ⇒ my1 = m2x1 + a or m2x1 - my1 + a = 0. (1) This is m a quadratic equation in m. Therefore, there are two values of m and for each value of m there is a tangent. Hence, there are two tangents from a given point to the parabola.

Note 6.7.1: If m1, m2 are the slopes of the two tangents then they are the roots of equation (6.3).

m1 + m2 = −

y1 x1

and m1m2 = a x

6.8  perpendicular tangents Show that the locus of the point of intersection of perpendicular tangents to a parabola is the directrix.

Solution Let the equation of the parabola be y2 = 4ax. Let (x1, y1) be the point of intersection of the two tangents to the parabola. Any tangent to the parabola is

y = mx + a m

If this tangent passes through (x1, y1) then y1 = mx1 + a . m (i.e.)  m2x1 - my1 + a = 0

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(6.4)

(6.5)

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6-10

Analytical Geometry

If m1, m2 are the slopes of the two tangents from (x1, y1), then they are the roots of equation (6.5). Since the tangents are perpendicular,

m1 ⋅ m2 = −1 ⇒ a = −1 or x1 + a = 0 x1

Therefore, the locus of (x1, y1) is x + a = 0, which is the directrix. Show that the locus of the point of intersection of two tangents to the parabola that make complementary angles with the axis is a line through the focus.

Solution Let (x1, y1) be the point of intersection of tangents to the parabola y2 = 4ax. Any tangent to the parabola is y = mx + a . If this line passes through (x1, y1), m then y1 = mx1 + a ⇒ m2 x1 − my1 + a = 0 . If m1, m2 are the slopes of the two m tangents, then y m1 + m2 = 1 , m1m2 = a x1 x1 If the tangents make complementary angles with the axis of the parabola, then m1 = tanq  and m2 = tan(90 - q ). m1m2 = tanq  × tan(90 - q ) = tanq  × cotq  = 1

⇒ a = 1 or x1

x1 − a = 0 .

The locus of the point of intersection of the tangents is x - a = 0, which is a straight line through the origin.

6.9  equation of tangent Find the equation of the tangent at (x1, y1) to the parabola y2 = 4ax. Let P(x1, y1) and Q(x2, y2) be two points on the parabola y2 = 4ax. Then

y12 = 4 ax1

(6.6)



y = 4 ax2

(6.7)

2 2

P (x1, y1)

Q (x2, y2)

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Parabola

6-11

The equation of the chord joining the points (x1, y1) and (x2, y2) is y − y1 y1 − y2 = x − x1 x1 − x2



From equations (6.6) and (6.7), we get

y1 − y2 = 4a x1 − x2 y1 + y2

y12 − y22 = 4 a( x1 − x2 ) or

Hence, the equation of the chord PQ is

y − y1 4 a( x − x1 ) = 4 a   or  y − y1 = x − x1 y1 + y2 y1 + y2

When the point Q(x2, y2) tends to coincide with P(x1, y1), the chord PQ b­ ecomes the tangent at P. Hence, the equation of the tangent at P is



y − y1 = 2a ( x − x1 ) or y1

yy1 − y12 = 2ax − 2ax1

yy1 = 2ax − 2ax1 + y12 = 2ax − 2ax2 + 4 ax1 (i.e) yy1 = 2a( x + x1 )

Aliter: The equation of the parabola is y2 = 4ax. Differentiating this equation with respect to x1 , we get 2 y ⇒



dy = 4a dx

dy 2a = dx y

 dy  = 2a = slope of the tangent at ( x1 , y1 )  dx  y1 at ( x1 , y1 )

The equation of the tangent at (x1, y1) is

y − y1 = 2a ( x − x1 ) ⇒ yy1 − y12 = 2ax − 2ax1 y1 or yy1 = 2ax − 2ax1 + 4 ax1 = 2a( x + x1 ) ∴ yy1 = 2a( x + x1 )

6.10  equation of normal Find the equation of the normal at (x1, y1) on the parabola y2 = 4ax.

Solution The slope of the tangent at (x1, y1) is 2a . y1

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6-12

Analytical Geometry

−y Therefore, the slope of the normal at (x1, y1) is 1 . 2a The equation of the normal at (x1, y1) is

y1 ( x − x1 ) ⇒ 2ay − 2ay1 = − xy1 + x1 y1 2a or xy1 + 2ay = 2ay1 + y1 x1

y − y1 = −

6.11  equation of chord of contact Find the equation of the chord of contact of tangents from (x1, y1) to the ­parabola y2 = 4ax. Q(x2, y2)

P(x1, y1) R(x3, y3)

Solution Let QR be the chord of contact of tangents from P(x1, y1). Let Q and R be the points (x2, y2) and (x3, y3), respectively. Then, the equation of tangents at Q and R are

yy2 = 2a (x + x2)

(6.8)



yy3 = 2a (x + x3)

(6.9)

These two tangents pass through P(x1, y1).

\ y1 y2 = 2a (x1 + x2)

(6.10)



y1 y3 = 2a (x1 + x3)

(6.11)

These two equations show that the points (x2, y2) and (x3, y3) lie on the line yy1 = 2a(x + x1). Therefore, the equation of the chord of contact of tangents from P(x1, y1) is yy1 = 2a(x + x1).

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Parabola

6-13

6.12  polar of a point Find the polar of the point with respect to the parabola y2 = 4ax.

Definition 6.12.1 The polar of a point with respect to a parabola is defined as the locus of the point of intersection of the tangents at the extremities of a chord passing through that point. Solution Let P(x1, y1) be the given point. Let QR be a variable chord passing through P. Let the tangents at Q and R intersect at (h, k). Then the equation of the chord of contact of tangents from (h, k) is yk = 2a(x + h). This chord passes through P(x1, y1).

\ y1k = 2a (x1 + h)

Then the locus of (h, k) is yy1 = 2a(x + x1) Hence, the polar of (x1, y1) with respect to y2 = 4ax is yy1 = 2a(x + x1)



Note 6.12.1: Point P is the pole of the line yy1 = 2a(x + x1). Note 6.12.2: Find the pole of the line lx + my + n = 0 with respect to the parabola y2 = 4ax. Let (x1, y1) be the pole. Then the polar of (x1, y1) is yy1 = 2a(x + x1)

(i.e.)  2ax - yy1 + 2ax1 = 0

But the polar of (x1, y1) is given  by lx + my + n = 0

(6.12) (6.13)

Equations (6.12) and (6.13) represent the same line. Then, identifying these two equations, we get 2a = − y1 = −2ax1 l m n n − ∴ x1 = , y = −2am l 1 l

(

)

Hence, the pole of the line is − n , −2am . l l

6.13  conjugate lines Definition 6.13.1 Two lines are said to be conjugate to each other if the pole of each lies on the other.

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6-14

Analytical Geometry

Find the condition for the lines lx + my + n = 0 and l1 x + m1 y + n1 = 0 to be conjugate lines with respect to the parabola y2 = 4ax.

Solution Let (x1, y1) be the pole of the lines l1 x + m1 y + n1 = 0 with respect to the parabola. The polar of (x1, y1) with respect to the polar y2 = 4ax is lx + my + n = 0. The equation of the polar of (x1, y1) with respect to the parabola y2 = 4ax is yy1 = 2a(x + x1) ⇒ 2ax - yy1 + 2ax1 = 0

(6.14)

But the polar of (x1, y1) is given by  lx + my + n = 0

(6.15)



Equations (6.14) and (6.15) represent the same line. Identifying these two equations, we get 2a = − y1 = 2ax1 l m n

(

)

The pole of the line lx + my + n = 0 is n , −2am . l l Since the lines lx + my + n = 0 and l1 x + m1 y + n1 = 0 are conjugate to each other, the pole of lx + my + n = 0 will lie on l1 x + m1 y + n1 = 0.

() (

)

l1 n + m1 −2am + n1 = 0. l l (i.e.)  l1n + l1n = 2amm1. This is the required condition.

6.14  PAIR of tangents Find the equation of pair of tangents from (x1, y1) to the parabola y2 = 4ax.

Solution x − x1 y − y1 = = r (6.16) cos q sin q Any point on this line is (x1 + rcosq, y1 + rsinq ). The points of intersection of the line and the parabola are given by (y1 + rsinq )2 = 4a(x1 + cosq )

The equation of a line through (x1, y1) is 



(i.e.) r 2 sin 2 q + 2r ( y1 sin q − 2a cos q ) + y12 − 4 ax1 = 0

(6.17)

The two values of r of this equation are the distances of point (x, y) to the point (x1, y1). If line (6.16) is a tangent to the parabola, then the two values of r must be equal and the condition for this is the discriminant of quadratic (6.17) is zero.

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Parabola

6-15

∴ 4( y1 sin q − 2a cos q )2 = 4 sin 2 q ( y12 − 4 ax1 )



Eliminating q  in this equation with the help of (6.16), we get 2

2

  y − y1   y − y1   x − x1   2 = 4 − 2a  4  y1    ( y1 − 4 ax1 )   r r r        (i.e.) ( yy1 − y12 − 2ax + 2ax1 )2 = ( y − y1 )2 ( y12 − 4 ax1 ) [ yy1 − 2a( x + x1 ) − y12 + 4 ax1 ]2 = ( y 2 + y12 − 2 yy1 )( y12 − 4 x12 )

{

}

= y 2 − 4 ax + y12 − 4 ax1 − 2 yy1 − 4 a( x + x1 ) × ( y12 − 4 ax1 ) (i.e.) (T − S1 )2 = ( S + S1 − 2T )S1



or T 2 + S12 − 2TS1 = SS1 + S12 − 2TS1 . (i.e.) T 2 = SS1 Therefore, the equation of pair of tangents from (x1, y1) is

[yy1 - 2a(x + x1)]2 = (y2 - 4ax) (y12 - 4ax1).

6.15  Chord in terms of mid-point Find the equation of a chord of the parabola in terms of its middle point (x1, y1). P

r R

(x1, y1)

Q

Solution Let the equation of the chord be x − x1 y − y1 = =r cos q sin q

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6-16

Analytical Geometry

Any point on this line is (x1 + rcosq, y1 + rsinq ). When the chord meets the parabola y 2 = 4ax, this point lies on the curve.

\ (y1 + rsinq )2 = 4a (x1 + rcosq )



(i.e.) r 2 sin 2 q + 2r ( y1 sin q − 2a cos q ) + y12 − 4 ax1 = 0

The two values of r are the distances RP and RQ, which are equal in magnitude but opposite in sign. The condition for this is the coefficient of r is equal to zero. ∴ y1 sin q − 2a cos q = 0 (i.e.)

y1 ( y − y1 ) 2a( x − x1 ) − =0 r r yy1 − y12 = 2ax − 2ax1 or

yy1 − 2a( x + x1 ) = y12 − 4ax1 (i.e.) T = S1

This is the required equation of the chord.

6.16  PARAMETRIC REPRESENTATION x = at2, y = 2at satisfy the equation y2 = 4ax. This means (at2, 2at) is a point on the parabola. This point is denoted by ‘t’ and t is called a parameter.

6.17  Chord joining two points Find the equation of the chord joining the points (at12, 2at1) and (at22, 2at2) on the parabola y2 = 4ax.

Solution The equation of the chord joining the points is y − 2at1 2at1 − 2at2 2a(t1 − t2 ) = = = 2 2 2 2 a ( t t1 + t2 x − at1 at1 − at2 1 − t 2 )(t1 + t 2 )

∴ y(t1 + t2 ) − 2at1 (t1 + t2 ) = 2 x − 2at12 (i.e.) y(t1 + t2 ) = 2 x − 2at12 + 2at12 + 2at1t2



y(t1 + t2 ) = 2 x + 2t1t2

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Parabola

6-17

Note 6.17.1: The chord becomes the tangent at ‘t’ if t1 = t2 = t. Therefore, the equation of the tangent at t is



y(2t ) = 2 x + 2at 2 or yt = x + at 2

6.18  Equations of tangent and normal Find the equation of the tangent and normal at ‘t’ on the parabola y2 = 4ax.

Solution The equation of the parabola is y2 = 4ax. Differentiating with respect to x, 2y

dy dy 2a = 4 a (ie) = dx dx y

 dy  2 2a 1  dx  at (at , 2at ) = 2at = t = slopee of the tangent at t

The equation of the tangent at t is

y − 2at = 1 ( x − at 2 ) t 2 yt − 2at = x − at 2 (i.e.)

yt = x + at 2

The slope of the normal at t is -t. The equation of the normal at ‘t’ is

y − 2at = −t ( x − at 2 ) (i.e.) y − 2at = − xt + at 3 (i.e.) y + xy = 2at + at 3

6.19  Point of intersection of tangents Find the point of intersection of tangents at t1 and t2 on the parabola y2 = 4ax.

Solution The equation of tangents at t1 and t2 are yt1 = x + at12



yt2 = x + at22

Subtrating, y(t1 − t2 ) = a(t12 − t22 )

(i.e.)  y = a (t1 + t2).

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6-18

Analytical Geometry

\ a(t1 + t2) t1 = x + at12 \ x = at1t2 Hence, the point of intersection is [at1t2, a(t1 + t2)].

6.20  Point of intersection of normals Find the point of intersection of normals at t1 and t2.

Solution The equation of normals at t1 and t2 are y + xt1 = 2at1 + at13



y + xt2 = 2at2 + at23

Subtracting, x(t1 − t2 ) = 2a(t1 − t2 ) + a(t13 − t23 )

⇒ x = 2a + a(t12 + t1t2 + t22 )



∴ y = − at1t2 (t1 + t2 )

Hence, the point of intersection of normals is

[2a + a(t12 + t1t2 + t22 ), − at1t2 (t1 + t2 )]

6.21  Number of normals from a point Show that three normals can always be drawn from a given point to a parabola.

Solution Let the equation of the parabola be y 2 = 4 ax. The equation of the normal at t is y + xt = 2at + at 3



If this passes through ( x1 , y1 ) then

y1 + x1t = 2at + at 3



∴ at 3 − t (2a − x1 ) − y1 = 0

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(6.18)

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Parabola

6-19

This being a cubic equation in t, there are three values for t. For each value of t there is a normal from( x1 , y1 ) to the parabola y 2 = 4 ax .

Note 6.21.1: If t1 , t2 , t3 are the roots of equation (6.18), then

t1 + t2 + t3 = 0 2a − x1 t1t2 + t2 t3 + t3t1 = a y t1t2 t3 = 1 a

(6.19) (6.20) (6.21)

Note 6.21.2: From (6.18), 2at1 + 2at2 + 2at3 = 0

Therefore, the sum of the coordinates of the feet of the normal is always zero.

6.22  intersection of a parabola and a circle Prove that a circle and a parabola meet at four points and show that the sum of the ordinates of the four points of intersection is zero.

Solution Let the equation of the circle be

x 2 + y 2 + 2 gx + 2 fy + c = 0

(6.22)

Let the equation of the parabola be y 2 = 4 ax



(6.23)

Any point on the parabola is (at2, 2at). When the circle and the parabola intersect, this point lies on the circle,

∴ ( a 2 t 2 )2 + (2at )2 + 2 g ( at 2 ) + 2 f (2at ) + c = 0 (i.e.) a 2 t 4 + ( 4 a 2 + 2 ga)t 2 + 4 fat + c = 0

(6.24)

This being a fourth degree equation in t, there are four values of t. For each value of t there is a point of intersection. Hence, there are four points of ­intersection of a circle and a parabola. If t1 , t2 , t3 , t 4 be the four roots of equation (6.24), then t1 + t2 + t3 + t 4 = 0 (6.25) Multiplying equation (6.25) by 2a, we get

2at1 + 2at2 + 2at3 + 2at 4 = 0

Therefore, the sum of the ordinates of the four points of intersection is zero.

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6-20

Analytical Geometry

ILLUSTRATIVE EXAMPLES BASED ON TANGENTS AND NORMALS Example 6.3 Find the equations of the tangent and normal to the parabola y2 = 4(x - 1) at (5, 4).

Solution y2 = 4(x - 1) Differentiating with respect to x, dy dy 2y =4⇒ = 4 =2 dx dx 2 y y  dy  = 2 = 1 = Slope of the tangent at (5, 4)  dx  4 2 at( 5, 4 ) \ The equation of the tangent at (5, 4) is y − 4 = 1 ( x − 5). 2 2y - 8 = x - 5 or x - 2y + 3 = 0. The slope of the normal at (5, 4) is -2. \ The equation of normal at (5, 4) is y - 4 = -2(x - 5) or 2x + y = 14.

Example 6.4 Find the condition that the straight line lx + my + n = 0 is a tangent to the parabola.

Solution Any straight line tangent to the parabola y 2 = 4 ax is of the form y = mx + c if c = a . m Consider the line lx + my + n = 0 (i.e.) my = −lx − n



(6.26)

⇒ y = −l x − n m m 2 If this is a tangent to the parabola, y = 4 ax then − n = − a m ( l /m )



⇒ − n = − am (i.e.) am2 = nl m l

Example 6.5 A common tangent is drawn to the circle x 2 + y 2 = r 2 and the parabola y 2 = 4 ax . Show that the angle q  which it makes with the axis of the parabola r 2 + 4a2 − r . is given by tan 2 q = 2r

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Parabola

6-21

Solution Let y = mx + c be a common tangent to the parabola y 2 = 4 ax 

and the circle

(6.27)

x 2 + y 2 = r 2 If y = mx + c is tangent to the parabola (6.27) then c= a m (i.e.) y = mx + a  m If y = mx + c is a tangent to the circle (6.29) then y = mx + r 1 + m2 



(6.28)

(6.29)

(6.30)

Equations (6.29) and (6.30) represent the same straight line. Identifying we get a = r 1 + m2 ⇒ a 2 = m2 r 2 (1 + m2 ) m



r 2 m 4 + r 2 m2 − a2 = 0



− r 2 ± r 4 + 4a2 r 2 − r 2 ± r r 2 + 4a2 = 2r 2 2r 2 2 Since m has to be positive, m2 =



m2 =

− r + r 2 + 4a2 or tan 2 q = 2r

r 2 + 4a2 − r  2r

or

Example 6.6 A straight line touches the circle x 2 + y 2 = 2a 2 and the parabola y 2 = 8ax. Show that its equation is y = ±( x + 2a) .

Solution The equation of the circle is x 2 + y 2 = 2a 2  The equation of the parabola is

(6.31)

y 2 = 4 ax 

(6.32)



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6-22

Analytical Geometry

A tangent to the parabola (6.32) is A tangent to the circle (6.33) is

y = mx + 2a m

(6.33)

y = mx + 2a 1 + m2 

(6.34)

Equations (6.33) and (6.34) represent the same straight line. Identifying we get,



2a 1 + m 2 = 2a m 2a 2 m2 (1 + m2 ) = 4 a 2 or m 4 + m2 − 2 = 0



( m2 − 1)( m2 + 2) = 0

m2 = 1 or -2; m2 = -2 is impossible. ∴ m2 = 1 or m = ±1 \ The equation of the common tangent is y = ± x ± 2a.

∴ y = ± ( x + 2a)

Example 6.7 Show that for all values of m, the line y = m( x + a) + a will touch the ­parabola m y 2 = 4 a( x + a). Hence show that the locus of a point,the two tangents form which to the parabolas y 2 = 4 a( x + a) and y 2 = 4b( x + b) one to each are at right angles, is the line x + a + b = 0.

Solution

y 2 = 4 a( x + a) y = m( x + a) + a  m

(6.35) (6.36)

Solving (6.35) and (6.36), we get their points of intersection. The x-coordinates of their points of intersection are given by, 2



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 m( x + a) + a  = 4 a( x + a)  m 

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Parabola

6-23

2 ∴ m2 ( x + a)2 + a 2 + 2a( x + a) = 4 a( x + a) m



2

or  m( x + a) − a  = 0  m  ∴ x + a = a2 . m



\ The two values of x and hence of y of the points of ­intersection are the same. Hence, y = m( x + a) + a is a tangent to the parabola m y 2 = 4 a( x + a). Let (x1, y1) be the point of intersection of the two tangents to the parabola y 2 = 4 a( x + a), y 2 = 4 a( y + b). The tangents are y = m( x + a) + a and y = m1 ( x + b) + b . Since they pass through (x1, y1), m m1 we have y1 = m( x1 + a) + a  (6.37) m y1 = m1 ( x1 + b) + b  (6.38) m1 Since the tangents are at right angles, m1m2 = −1. Subtracting (6.38) from (6.37), we get   0 = x1 ( m − m1 ) + a m + 1 − b  m1 + 1  m m1  

( ) 0 = x (m + 1 ) + a (m + 1 ) + b (m + 1 ) m m m

since m1 = −1 m

1

(

)

Cancelling m + 1 , x1 + a + b = 0 m The locus of (x1, y1) is x + a + b = 0.

Example 6.8 Prove that the locus of the point of intersection of two tangents to the parabola y 2 = 4 ax , which makes an angle of a  with x-axis, is y 2 − 4 ax = ( x + a)2 tan 2 a . Determine the locus of point of intersection of perpendicular tangents.

Solution Let (x1, y1) be the point of intersection of tangents. Any tangent to the ­parabola is y = mx + a . If this passes through (x1, y1) then y1 = mx1 + a . m m

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6-24



Analytical Geometry

(i.e.) m2 x1 − my1 + a = 0 . m1 + m2 =



m1m2 = a x1



tan 2 a =





y1 x1 Now tana =

m1 − m2 1 + m1m2 y12 −4 a x1 x12

( m1 + m2 )2 − 4m1m2 y 2 − 4 ax1 = = 1 2 2 ( x1 + a)2 (1 + m1m2 )  a 1 +  x  1

⇒ ( x1 + a)2 tan 2 a = y12 − 4 ax1

\ The locus of (x1, y1) is y 2 − 4 ax = ( x + a)2 tan 2 a . If the tangents are perpendicular, tan a = tan 90° = É \ The locus of perpendicular tangents is directrix.

Example 6.9 Prove that if two tangents to a parabola intersect on the latus rectum produced then they are inclined to the axis of the parabola at complementary angles.

Solution Let (x1, y1) the equation of the parabola be y 2 = 4 ax . Let y = mx + a be any m tangent to the parabola. Let the two tangents intersect at (a, y1), a point on the latus rectum. Then (a1, y1) lies, on y = mx + a . m

∴ y1 = ma + a or m2 a − my1 + a = 0 m

(6.39)

If m1 and m2 are the slopes of the two tangents to the parabola then m1m2 = 1. (i.e.) tan q ⋅ tan(90 − q ) = 1. (i.e.) The tangent makes complementary angles to the axis of the parabola.

Example 6.10 Prove that the locus of poles of the chords of the parabola y 2 = 4 ax which subtends a constant angle a  at the vertex is the curve ( x + 4 a)2 tan 2 a = 4( y 2 − 4 ax ).

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Parabola

6-25

Solution Let (x1, y1) be the pole of a chord of the parabola. Then the polar of (x1, y1) is yy1 = 2a( x + x1 )



(6.40)

Q

A

a

R

which is the chord of contact from (x1, y1). The combined equation of the lines AQ and AR is got by homogenization of the equation of the parabola y 2 = 4 ax with the help of (6.40). \ The combined equation of the lines is  yy − 2ax  2 2 2 2 y 2 = 4 ax  1  (i.e.)  x1 y = 2 xyy1 − 4 ax or 4 ax − 2 x1 xy + x1 y = 0 2 ax   1 Since QAR = a, tan a =



4( h2 − ab) 2 h2 − ab , ⇒ tan 2 a = a+b ( a + b) 2 ⇒ tan 2 a =

4( y12 − 4 ax1 ) ( x1 + 4 a)2

The locus of (x1, y1) is ( x + 4a)2 tan 2 a = 4( y 2 − 4ax ). Note 6.10.1: If a  = 90°, the locus of (x1, y1) is x + 4a = 0. Example 6.11 If two tangents are drawn to a parabola making complementary angles with the axis of the parabola, prove that the chord of contact passes through the point where the axis cuts the directrix.

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6-26

Analytical Geometry

Solution Let y = mx + a be a tangent from a point (x1, y1) to the parabola y 2 = 4 ax . m Then y1 = mx1 + a or m2 x1 − my1 + a = 0 . If the tangents make complemenm tary angles with the axis of the parabola then m1m2 = 1 or a = 1 ⇒ x1 = a. x1 The equation of the chord of contact from (a, y1) to the parabola is yy1 = 2a(x + a). When the chord of contact meets the x-axis, y = 0. \  x + a = 0 or x = -a. \ The chord of contact passes through the point (-a, 0) where the axis cuts the directrix.

Example 6.12 Find the locus of poles of tangents to the parabola y 2 = 4 ax with respect to the parabola x 2 = 4by .

Solution Let (x1, y1) be the pole with respect to the parabola x 2 = 4by . Then the polar xx of (x1, y1) is xx1 = 2b(y + y1), y = 1 − y1. This is a tangent to the parabola 2b y2 = 4ax. The condition for tangent is c = a (i.e.) − y1 = 2ab or x1y1 + m x1 2ab = 0. The locus of (x1, y1) is the straight line xy + 2ab = 0.

Example 6.13 From a variable point on the tangent at the vertex of a parabola, the perpendicular is drawn to its polar. Show that the perpendicular passes through a fixed point on the axis of the parabola.

Solution The equation of the tangent at the vertex is x = 0. Any point on this line is (0, y1). The polar of (0, y1) with respect to the parabola y 2 = 4 ax is yy1 = 2ax. The equation of the perpendicular to the polar of (x1, y1) is y1x + 2ay = k. This passes through (0, y1). \k = 2ay1. \ The equation of the perpendicular to the polar from (0, y1) is y1x + 2ay = 2ay1, when this line meets the x-axis, y1x = 2ay1 or x = 2a

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Parabola

6-27

Hence, the perpendicular passes through the point (2a, 0), a fixed point on the axis of the parabola.

Example 6.14 The polar of any point with respect to the circle x2 + y2 = a2 touches the ­parabola y 2 = 4 ax . Show that the point lies on the parabola y 2 = − ax .

Solution The polar of the point (x1, y1) with respect to the circle x2 + y2 = a2 is xx1 + yy1 = a2

(i.e.)

y=−

2 x1 x+ a y1 y1

This is a tangent to the parabola y 2 = 4 ax . The condition for that is c = a . m

(i.e.)

a 2 = a  − y1  or ax = − y 2 1 1  x  y1 1

The locus of (x1, y1) is y2 = -ax which is a parabola.

Example 6.15 Find the locus of poles with respect to the parabola y 2 = 4 ax of tangents to the circle x2 + y2 = c2.

Solution Let the pole with respect to the parabola y 2 = 4 ax be (x1, y1). Then the polar of (x1, y1) is

yy1 = 2a(x + x1) 2 ax 1 (i.e.) y = 2a x + . This is a tangent to the circle x2 + y2 = a2. The condition y1 y1 for this is ‘c2 = a2 (1 + m2).



(i.e.)

2  4 a 2 x12 = c 2 1 + 4 a2  2 y y1  

⇒ 4 a 2 x12 = c 2 ( y12 + 4 a 2 ).

The locus of (x1, y1) is 4a2x2 = c2(y2 + 4a2).

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6-28

Analytical Geometry

Example 6.16 A point P moves such that the line through it perpendicular to its polar with respect to the parabola y 2 = 4 ax touches the parabola x 2 = 4by . Show that the locus of P is 2ax + by + 4a2 = 0.

Solution Let P be the point (x1, y1). The polar of P with respect to y 2 = 4 ax is yy1 = 2a(x + x1) or 2ax - yy1 + 2ax1 = 0



(6.41)

The equation of the perpendicular to (6.41) is y1x + 2ay + k = 0. This passes through (x1, y1). \ x1y1 + 2ay1 + k = 0. ⇒ k = -(x1y1 + 2ay1) Hence, the equation of the perpendicular is y1x + 2ay - (x1y1 + 2ay1) = 0. (i.e.)

x y + 2ay1 x = −2a y + 1 1 ⇒ x = −2a y + ( x1 + 2a) y1 y1 y1

This is a tangent to the parabola x2 = 4by. −by1 \ The condition x1 + 2a = 2a \ The locus of (x1, y1) is 2ax + by + 4a2 = 0.

Example 6.17 If the polar of the point P with respect to a parabola passes through Q then show that the polar of Q passes through P.

Solution Let the equation of the parabola be y 2 = 4 ax . Let P and Q be the points (x1, y1) and (x2, y2). Then the polar of P is yy1 = 2a(x + x1). Since this passes through Q(x2, y2), we get y1y2 = 2a(x1 + x2). This condition shows that the point (x1, y1) lies on the line yy2 = 2a(x + x2). \ The polar of Q passes through the point P(x1, y1).

Example 6.18 P is a variable point on the tangent at the vertex of the parabola y 2 = 4 ax . Prove that the locus of the foot of the perpendicular from P on its polar with respect to the parabola is the circle x2 + y2 - 2ax = 0.

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Parabola

6-29

Solution P is the variable point on the tangent at the vertex of the parabola y 2 = 4 ax . The equation of the tangent at the vertex is x = 0. Any point on the tangent at the vertex is P (0, y1). The polar of (0, y) is yy1 = 2ax



(6.42)

The equation of the perpendicular to this polar is y1x + 2ay = k



(6.43)

This passes through (0, y1).

\ 2ay1 = k.

\The equation of the perpendicular from P to its polar is y1x + 2ay = 2ay1



(6.44)

Let (l, m) be the point of intersection of (6.42) and (6.43). Then 2al - my1 = 0. y1l + 2am - 2ay1 = 0

Solving



Now,

l = m = 1 2ay12 4 a 2 y1 4 a 2 + y12 ∴l = l 2 + m2 − 2am =

2ay12 4a2 y ,m = 2 1 2 2 4 a + y1 y + 4a 2 1

4 a 2 y14 + 16a 4 y12 4 a 2 y12 − 2 2 2 2 y1 + 4 a 2 ( y1 + 4 a )

=

4 a 2 y12 ( y12 + 4 a 2 ) 4 a 2 y12 − 2 2 2 2 ( y1 + 4 a ) y1 + 4 a 2

=

4 a 2 y12 4 a 2 y12 − =0 y12 + 4 a 2 y12 + 4 a 2



\ The locus of (l, m) is x2 + y2 - 2ax = 0.

Example 6.19 If from the vertex of the parabola y 2 = 4 ax , a pair of chords can be drawn at right angles to one another and with these chords as adjacent sides a rectangle be made, prove that the locus of further angle of the rectangle is the parabola y2 = 4a(x - 8a).

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6-30

Analytical Geometry

Solution P

R

A

Q

Let AP and AQ be the chords of the parabola such that PAQ = 90°. Complete the rectangle APRQ. Then the midpoints of AR and PQ are the same. Let the equations of AP be y = mx. Solving y = mx and y2 = 4ax, we get m2x2 = 4ax or x = 4 a2 . m \ The point P is  4 a , 4 a  . Since AQ is perpendicular to AP, slope of AQ  m2 m  is −1 . m Hence, the point Q is (4am2, -4am). x y  Let (x1, y1) be the point R. The midpoint of AR is  1 , 1  . The midpoint of  2 2   2 1   PQ is 2a  m + 2  ; −2a m − 1  . Since the midpoint of AR is the same as m  m   

(

)

that of PQ, x1 = 2a  m2 + 12  ,  2 m 





∴

(

x1  = 2a  m − 1 2 m 

)

2

(

y1 = −2a m − 1 2 m

)

 y2  y2  + 2 or x1 = 4 a  1 2 + 2 = 1 + 8a  16a  4a

Hence, the locus of (x1, y1) is y2 = 4a(x - 8a).

Example 6.20 Show that if r1 and r2 be the lengths of perpendicular chords of a parabola

(

2

2

)

drawn through the vertex then ( r1r2 )2 = 16a 2 r1 3 + r2 3 .

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6-31

Parabola

Solution AP = r1, AQ = r2, PAX = q , QAX = 90 − q



y

P r1 (x, y)

A

q 90 − q

r2

X

x

Q

The coordinates of P are (r1cosq, r1sinq ). The coordinates of Q are (r2sinq, r2cosq ). Since P lies on the parabola y2 = 4ax,

q r12 sin 2 q = 4 a ⋅ r1 cos q ∴ r1 = 4 a cos sin 2 q

Similarly,

( 4 a) 2 q ; r r = 16a 2 = r2 = 4 a sin 1 2 2 cos q sin q (cos q sin q ) cos q ( r1r2 ) 4 / 3 =



Also

( 4 a )8 / 3  (cos q sin q ) 4 / 3

(6.45)

 ( 4 a cos q )2 / 3 ( 4 a sin q )2 / 3  RHS = 16a 2 ( r12 / 3 + r22 / 3 ) = 16a 2  +  2 2/3 (cos 2 q )2 / 3   (sin q )



 ( 4 a)2 / 3 (cos2 q + sin 2 q )  ( 4 a )8 / 3 = 16a 2  (6.46) =  4/3 (cosq sin q ) 4 / 3   (cos q sin q )

From equations (6.45) and(6.46), ( r1r2 ) 4 / 3 = 16a 2 ( r12 / 3 + r22 / 3 ).

Example 6.21 Show that the latus rectum of a parabola bisects the angle between the tangents and normal at either extremity.

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6-32

Analytical Geometry

Solution L 45

(a,

2a

45

45 B

S (a, 0)

A

)

135

M

x

Let LSL′ be the latus rectum of the parabola y2 = 4ax. The coordinates of L are (a, 2a). The equation of tangent at L is y ⋅ 2a = 2a( x + a) y = x + a (6.47) The slope of the tangent is 1. ∴ The slope of the normal at L is -1. LS is perpendicular to x-axis. ∴ BLS = SLM = 45°.



∴ Latus rectum bisects the angle between the tangents and normal at L.

Example 6.22 Show that the locus of the points of intersection of tangents to y2 = 4ax which intercept a constant length d on the directrix is(y2 - 4ax) (x + a)2 = d 2x2.

Solution y Q ) , y1 P(x 1

L A M

x

R

x+a=0

Let P(x1, y1) be the point of intersection tangent to the parabola. Then the equation of the pair of tangents PQ and PR is T 2 = SS1. (i.e.) [ yy1 − 2a( x + x1 )]2 = ( y 2 − 4 ax )( y12 − 4 ax1 ) . When these lines meet the directrix x = -a, we have

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Parabola

6-33

[ yy1 − 2a( − a + x1 )]2 = ( y 2 + 4 a 2 )( y12 − 4 ax1 )



(i.e.)

y 2 [ y12 − y12 + 4 ax1 ] − y[4 a( − a + x1 ) y1 ] + [4 a 2 ( − a + x1 )2 − 4 a 2 ( y12 − 4 ax1 )] y = 0.



x1 y 2 − y1 ( x1 − a) y + a[( x1 − a)2 − ( y12 − 4 ax1 )] = 0 x1 y 2 − y1 ( x1 − a) y + a[( x1 + a)2 − y12 ] = 0.

If y1 and y2 are the ordinates of the point of intersection of tangents with the directrix x + a = 0, then a ( x1 + a)2 − y12  ( x1 − a) y1 y1 + y2 = or y1 y2 = x1 x1 Then d 2 = ( y1 − y2 )2 = ( y1 + y2 )2 − 4 y1 y2 =

( x1 − a)2 y12 − 4ax1 ( x1 + a)2 − y12  x12

∴ d 2 x12 = ( x1 − a)2 y12 − 4 ax1 ( x1 + a)2 − y12 

= y12 ( x1 − a)2 + 4 ax1  − 4 ax1 ( x1 + a)2 = y12 ( x1 + a)2 − 4 ax1 ( x1 + a)2 = ( y12 − 4 ax1 )( x1 + a)2

\ The locus of (x1, y1) is (y2 - 4ax) (x + a)2 = d 2x2.

Example 6.23 Show that the locus of midpoints of chords of a parabola which subtend a right angle at the vertex is another parabola whose latus rectum is half the latus rectum of the parabola.

Solution P

A

(x1, y1)

90°

Q

Let the equation of the parabola be

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y2 = 4ax

(6.48)

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6-34

Analytical Geometry

Let (x1, y1) be the midpoint of the chord PQ. Then the equation of PQ is T = S1

(i.e.)  yy1 − 2a( x + x1 ) = y12 − 4 ax1



yy1 − 2ax = y12 − 2ax1

(6.49)

The combined equation of the lines AP and AQ is got by homogenization of equation (6.48) with the help of (6.49).  y y − 2ax  . \ The combined equation of OP and OQ is y 2 = 4 ax  12  y1 − 2ax1  ( y12 − 2ax1 ) y 2 − 4 ax( yy1 − 2ax ) = 0



Since PAQ = 90°, coefficient of x2 + coefficient of y2 = 0

(i.e.)  y12 − 2ax1 + 8a 2 = 0 ⇒ y12 = 2a( x1 − 4 a).

The locus of (x1, y1) is y2 = 2a(x - 4a) which is a parabola whose latus rectum is half the latus rectum of the given parabola.

Example 6.24 Show that the locus of midpoints of chords of the parabola of constant length 2l is (y2 - 4ax) (y2 + 4a2) + 4a2l2 = 0.

Solution P l R

(x1, y1) l

Q



Let (x1, y1) be the midpoint of a chord of the parabola y2 = 4ax

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(6.50)

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Parabola

6-35

Let the equation of the chord be x − x1 y − y1 = = r (6.51) cos q sin q Any point on this line is (x1 + r cosq, y1 + r sinq ). This point lies on the parabola y2 = 4ax. (i.e.) (y1 + r sinq )2 = 4a(x1 + r cosq ). r 2 sin 2 q + 2r ( y1 sin q − 2a cos q ) + y12 − 4 ax1 = 0



(6.52)

The two values of r are the distances RP and RQ which are equal in magnitude but opposite in sign. The condition for this is the coefficient of r = 0. (i.e.) y1sinq - 2a cosq = 0. y

1 2

2a

+

4a 2 q

y1

Then from Equation (6.50), r 2 sin 2 q + y12 − 4 ax1 = 0 2   r 2  2 4 a 2  + ( y12 − 4 ax1 ) = 0  y1 + 4 a 



The locus of (x1, y1) is (y2 - 4ax) (y2 + 4a2) + 4a2l 2 = 0. (since r = l)

Example 6.25 Show that the locus of the midpoints of focal chords of a parabola is another parabola whose vertex is at the focus of the given parabola.

Solution Let the given parabola be y2 = 4ax (6.53) Let (x1, y1) be the midpoint of a chord of this parabola. Then its equation is yy1 − 2ax = y12 − 2ax1. If this is a focal chord then this passes through (a, 0).

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∴ −2a 2 = y12 − 2ax1.

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6-36

Analytical Geometry

The locus of (x1, y1) is y 2 = 2a( x − a) which is a parabola whose vertex is at the focus of the given parabola.

Example 6.26 2 From a point common tangents are drawn to the circle x 2 + y 2 = a and 2 the parabola y2 = 4ax. Find the area of the quadrilateral formed by the ­common tangents, the chord of contact of the circle and the chord of contact of the parabola.

Solution N L

P (−a, 0)

A

D

M Q

Any tangent to the parabola y2 = 4ax is

y = mx + a m

(6.54)

2 If this is also a tangent to the circle x 2 + y 2 = a then a = a (1 + m2 ) 2 m 2 2 2 4 2 2 2 \ m (1 + m ) = 2 or m + m - 2 = 0 or (m - 1)(m + 2) = 0 ⇒ m2 = 1 or -2. But m2 = -2 is inadmissible since m2 has to be positive. \m = ±1. Hence the common tangents are y = ± (x + a). The two tangents meet at P (-a, 0). 2 The equation of the chord of contact from (-a, 0) to the circle x 2 + y 2 = a 2 2 is − xa = a or x = − a . 2 2 The equation of the chord of contact from (-a, 0) to the parabola y2 = 4ax is 0 = 2a(x - a) or x - a = 0. When x = a, y = ±2a. Hence N and Q are (a, 2a) and (a, -2a). When x = -a, y = ± a , 2

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Parabola

(

) (

6-37

)

\L and M are − a , a and − a , − a . 2 2 2 2 \ Area of quadrilateral LMQN = Area of trapezium LMQN = 1 ( LM + NQ ) ⋅ AD 2 Since PD = 2a, PA= a , 2 a ∴ AD = 2a − = 3a . 2 2 1 = ( a + 4 a) 3a 2 2 2 15 a = . 4

Example 6.27 The polar of a point P with respect to the parabola y2 = 4ax meets the curve in Q and R. Show that if P lies on the line lx + my + n = 0 then the locus of the middle point of the QR is l(y2 - 4ax) + 2a(lx + my + n) = 0.

Solution Let P be the point (h, k). The polar of P(h, k) with respect to the parabola y2 = 4ax is yk = 2a(x + h) (6.55) The polar of P meets the parabola y2 = 4ax at Q and R. Let P(x1, y1) be the midpoint of QR. Its equation is yy1 − 2ax = y12 − 2ax1



(6.56)

Equations (6.55) and (6.56) represent the same line. \Identifying equations (6.55) and (6.56) we get, k = 1 = 2ah y1 1 y12 − 2ax1

∴ y1 = k and y12 − 2ax1 = 2ah

(6.57)

Since the point (h, k) lies on lx + my + n = 0, lh + mk + n = 0. l ( y12 − 2ax1 ) + my1 + n = 0 Using equation (6.56), 2a (i.e.)  l ( y12 − 2ax1 ) + 2a( my1 + n) = 0 (i.e.)  l ( y12 − 4 ax1 ) + 2a(lx1 + my1 + n) = 0 \The locus of (x1, y1) is l(y2 - 4ax) + 2a(lx + my + n) = 0.

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6-38

Analytical Geometry

Example 6.28 Prove that area of the triangle inscribed in the parabola y2 = 4ax is 1 ( y − y )( y − y )( y − y ) where y , y and y are the ordinates of the 2 2 3 3 1 1 2 3 8a 1 ­vertices of the triangle.

Solution Let (x1, y1), (x2, y2) and (x3, y3) be the vertices of the triangle inscribed in the  y2   y2   y2  parabola y2 = 4ax. Then the vertices are  1 , y1  ,  2 , y2  and  3 , y3  .  4a   4a   4a  The area of the triangle is



=1 2

∑ x1 ( y2 − y3 ) = 12

y12 ∑ 4a ( y2 − y3 ) = 81a

∑y

2 1

( y2 − y3 )

= 1 y1 ( y2 − y3 ) . y2 ( y3 − y1 ) . y3 ( y1 − y2 ) 8a

Example 6.29 An equilateral triangle is inscribed in the parabola y2 = 4ax one of whose vertices is at the vertex of the parabola. Find its side.

Solution B y2

r A (0,0)

L r C

C

 3 r The coordinates of B are B(r cos 30°, rsin 30°),  r, .  2 2  Since this point lies on the parabola y2 = 4ax, then

r 2 = 4a ⋅ r 3 4 2

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∴ r = 8a 3

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6-39

Parabola

Exercises 1.

Show that two tangents can be drawn from a given point to a parabola. If the tangents make angles q1 and q2 with x axis such that (i) tanq1 + tanq2 is a constant show that the locus of point of intersection of tangents is a straight line through the vertex of a parabola. (ii) if tanq1 ⋅ tanq2 is a constant show that the locus of the point of intersecting is a straight line. (iii) if q1 + q2 is a constant show that the locus of the point of intersection of tangents is a straight line through the focus. (iv) if q1 and q2 are complementary angles then the locus of point of intersection is the straight line x = a.

2.

Find the locus of point of intersection of tangents to the parabola y2 = 4ax which includes an angle of p . 3 Ans.: 3(x + a)2 = y2 - 4ax

3.

Show that the locus of the poles of chords of the parabola y2 = 4ax which subtends an angle of 45° at the vertex is the curve (x + a)2 = 4(y2 - 4ax).

4.

Show that the locus of poles of all tangents to the parabola y2 = 4ax with respect to the parabola y2 = 4bx is the parabola ay2 = 4b2x.

5.

Show that the locus of poles of chords of the parabola which subtends a right angle at the vertex is x + 4a = 0.

6.

Show that if tangents be drawn to the parabola y2 = 4ax from any point on the straight line x + 4a = 0, the chord of contact subtends a right angle at the vertex of the parabola.

7.

Perpendiculars are drawn from points on the tangent at the vertex on their polars with respect to the parabola y2 = 4ax. Show that the locus of the foot of the perpendicular is a circle centre at (a, 0) and radius a.

8.

Show that the locus of poles with respect to the parabola y2 = 4ax of tangents to the circle x2 + y2 = 4a2 is x2 - y2 = 4a2.

9.

A point P moves such that the line through the perpendicular to its polar with respect to the parabola y2 = 4ax touches the parabola x2 = 4by. Show that the locus of P is 2ax + by + 4a2x = 0.

10.

If a chord of the parabola y2 = 4ax subtends a right angle at its focus, show that the locus of the pole of this chord with respect to the given parabola is x2 - y2 + 6ax + a2 = 0.

11.

Show that the locus of poles of all chords of the parabola y2 = 4ax which are at a constant distance d from the vertex is d2y2 + 4a2 (d2 - x2) = 0.

12.

Show that the locus of poles of the focal chords of the parabola y2 = 4ax is x + a = 0.

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Analytical Geometry

13.

If two tangents to the parabola y2 = 4ax make equal angles with a fixed line show that the chord of contact passes through a fixed point.

14.

Prove that the polar of any point on the circle x2 + y2 - 2ax - 3a2 = 0 with respect to the circle x2 + y2 + 2ax - 3a2 = 0 touches the parabola y2 = 4ax.

15.

Show that the locus of the poles with respect to the parabola y2 = 4ax of the tangents to the curve x2 - y2 = a is the ellipse 4x2 + y2 = 4ax.

16.

P is a variable point on the line y = b, prove that the polar of P with respect to the parabola y2 = 4ax is a fixed directrix.

17.

The perpendicular from a point O on its polar with respect to a parabola meet the polar in the points M and cuts the axis in G. The polar meets x-axis in T and the ordinate through O intersects the curve in P and P′. Show that the points G, M, P, P′ and T lie on a circle whose centre is at the focus S.

18.

Tangents are drawn to the parabola y2 = 4ax from a point (h, k). Show that the area of the triangle formed by the tangents and the chord of contact is ( k 2 − 4 ah)3 / 2 . a Prove that the length of the chord of contact of the tangents drawn from the 1 2 y1 + 4 a 2 ( y12 − 4 ax1 ) . Hence show point (x1, y1) to the parabola y2 = 4ax is a that one of the triangles formed by these tangents and their chord of contact is 1 2 ( y1 − 4 ax1 )3 / 2 . 2a Tangents are drawn from a variable point P to the parabola y2 = 4ax such that they form a triangle of constant area with the tangent at the vertex. Show that the locus of P is (y2 - 4ax)x2 = 4c2.

19.

20.

21.

Prove that the tangent to a parabola and the perpendicular to it from its focus meet on the tangent at the vertex.

22.

Show that a portion of a tangent to a parabola intercepted between directrix and the curve subtends a right angle at the focus.

23.

The tangent to the parabola y2 = 4ax make angles q1 and q2 with the axis. Show that the locus of the point of intersection such that cotq1 + cotq2 = c is y = ac.

24.

If perpendiculars be drawn from any two fixed points on the axis of a parabola equidistant from the focus on any tangent to it, show that the difference of their squares is a constant.

25.

Prove that the equation of the parabola whose vertex and focus on x-axis at distances 4a and 5a from the origin respectively (a > 0) is y2 = 4a(x - 4a). Also obtain the equation to the tangent to this curve at the end of latus rectum in the first quadrant. Ans.: y = x - a

26.

Chords of a parabola are drawn through a fixed point. Show that the locus of the middle points is another parabola.

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Parabola

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27.

Find the locus of the middle points of chords of the parabola y2 = 2x which touches the circle x2 + y2 - 2x - 4 = 0.

28.

A tangent to the parabola y2 + 4bx = 0 meets the parabola y2 = 4ax at P and Q. Show that the locus of the middle point of PQ is y2(2a + b) = 4a2x.

29.

Through each point of the straight line x - my = h is drawn a chord of the parabola y2 = 4ax which is bisected at the point. Prove that it always touches the parabola (y + 2am)2 = 8axh.

30.

Two lines are drawn at right angles, one being a tangent to the parabola y2 = 4ax and the other to y2 = 4by. Show that the locus of their point of intersection is the curve (ax + by) (x2 + y2) = (bx - ay)2.

31.

A circle cuts the parabola y2 = 4ax at right angles and passes through the focus. Show that the centre of the circle lies on the curve y2(a + x) = a(a + bx)2.

32.

Two tangents drawn from a point to the parabola make angles q1 and q2 with the x-axis. Show that the locus of their point of intersection if tan2q1 + tan2q2 = c is y2 - cx2 = 2ax.

33.

If a triangle PQR is inscribed in a parabola so that the focus S is the orthocentre and the sides meet the axes in points K, L and M then prove that SK ⋅ SL ⋅ SM − 4 SA2 = 0 where A is the vertex of the parabola.

34.

Chords of the parabola y2 = 4ax are drawn through a fixed point (h, k). Show  k2 k  that the locus of the midpoint is a parabola whose vertex is  h − ,  and 8a 2   latus rectum is 2a.

35.

Show that the locus of the middle points of a system of parallel chords of a parabola is a line which is parallel to the axis of the parabola.

36.

Show that the locus of the midpoints of chords of the parabola which subtends a constant angle a  at the vertex is ( y 2 − 2ax − 8a 2 )2 tan 2 a = 16a 2 ( 4 ax − y 2 ).

Illustrative Examples based on parameters Example 6.30 Prove that perpendicular tangents to the parabola will intersect on the ­directrix.

Solution Let the tangents at t1 and t2 intersect at P. The equation of tangents at t1 and t2 are yt1 = x + at12 and yt2 = x + at22. The slopes of the tangents are 1 and 1 . Since the tangents are perpendicular, t1 t2 1 ⋅ 1 = −1 t1 t2

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\t1t2 = −1

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6-42

Analytical Geometry

The point of intersection of the tangents at t1 and t2 is P(at1t2, a(t1 + t2)) (i.e.) (-a, a(t1 + t2)). This point lies on the line x + a = 0. \Perpendicular tangents intersect on the directrix.

Example 6.31 Prove that the tangents at the extremities of a focal chord intersect at right angles on the directrix.

Solution Let t1 and t2 be the extremities of a focal chord. Then the equation of the chord is y (t1 + t2) = 2x + 2at1t2. This passes through the focus (a, 0). ∴0 = 2a + 2at1t2 or t1t2 = -1



(6.58)

\Tangents at t1 and t2 are perpendicular. The point of intersection of tangents at t1 and t2 is [at1t2, a(t1 + t2)] (i.e.) (-a, a(t1 + t2)). This point lies on the directrix. Hence the tangents at the extremities of a focal chord intersect at right angles on the directrix.

Example 6.32 Prove that any tangent to a parabola and perpendicular on it from the focus meet on the tangent at the vertex.

Solution Let the equation of the parabola y2 = 4ax. The equation of the tangent at t is

yt = x + at2



y - 0 = -t(x - a) or y + xt = at

(6.59) 1 The slope of the tangent is . The slope of the perpendicular to it is -t. t Hence the equation of the perpendicular line passing through focus (a, 0) is (6.60)

Multiplying equation (6.60) by t, we get yt + xt2 = at2



(6.61)

Equation (6.59) – equation (6.61) gives x(1 + t2) = 0 or x = 0.

\y = at

Hence, the point of intersection of (6.59) and (6.60) is (0, at) and this point lies on y-axis.

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Example 6.33 Show that the orthocentre of the triangle formed by the tangents at three points on a parabola lies on the directrix.

Solution Let t1, t2 and t3 be points of contact of the tangents at the points A, B and C, respectively on the parabola y2 = 4ax, forming a triangle PQR. The equation of QR is yt3 = xt3 + at32 (6.62) A P Q

t1 t2

B L R

C t3

P is the point of intersection of tangents at t1 and t2. This point is [at1t2, a(t1 + t2)]. The slope of PL, perpendicular to QR is -t3. \ The equation of PL is y - a(t1 + t2) = -t3[x - at1t2] (i.e.)

y + xt3 = a(t1 + t2) + at1t2t3

(6.63)

Then the equation of QM perpendicular from Q on PR is

y + xt2 = a(t2 + t3) + at1t2t3

(6.64)

Equation (6.63) – equation (3) gives x(t3 - t2) = a(t2 - t3) or x = -a. This point lies on the directrix x + a = 0. Hence the orthocentre lies on the directrix.

Example 6.34 The coordinates of the ends of a focal chord of the parabola y2 = 4ax are (x1, y1) and (x2, y2). Prove that x1x2 = a2 and y1y2 = -4a2.

Solution Let t1 and t2 be the ends of a focal chord. Then the equation of the focal chord is y(t1 + t2) = 2x + at1t2. Since this passes through the focus (a, 0), 0 = 2a + at1t2 or t1t2 = -1.

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6-44

Analytical Geometry

x1 x2 = at12 ⋅ at22 = at12 t22 = a since t1t2 = −1



y1 y2 = 2at1 ⋅ 2at2 = −4 a 2

Example 6.35 A quadrilateral is inscribed in a parabola and three of its sides pass through fixed points on the axis. Show that the fourth side also passes through a fixed point on the axis of the parabola.

Solution Let t1, t2, t3 and t4 be respectively vertices A, B, C and D of the quadrilateral inscribed in the parabola y2 = 4ax. The equation of chord AB is y (t1 + t2) = 2x + 2at1t2



(6.65)

When this meets the x-axis y = 0 (i.e.) x = -at1t2 = k1. Since AB meets the x-axis at a fixed point, ∴ t1t2 =



Similarly,

t 2 t3 =

− k1 . a −k − k2 ,t t = 3 . a a 34

Multiplying these, we get



− k1k 2 k3 a3  k 2  −k k k t1t 4  22  = 1 32 3 a a  t1t22 t32 t 4 =

∴ t1t2 =

− k1k3 = a constant. k2 a

Hence, the fourth side of the quadrilateral also passes through a fixed point.

Example 6.36 Tangents to the parabola y2 = 4ax are drawn at points whose abscissae are in the ratio k:1. Prove that the locus of their point of intersection is the curve y 2 = ( k 1/ 4 + k −1/ 4 ) 2 x 2 .

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Solution Let the tangents at t1 and t2 intersect at P(x1, y1) at 2 Given that 12 = k at2 ⇒ t1 = kt2



(6.66)

t1 P(x1, y1)

t2

The point of intersection of the tangents at t1 and t2 is x1 = at1t2 and y1 = a(t1 + t2 ). x1 = k at22 and y1 = a( k + 1)t2



x1 = k ⋅ a ⋅



2

 k + 1 ⇒ y12 =  1/ 4  x1 2 2  k  a ( k + 1) y12

\ The locus of (x1, y1) is y 2 = ( k 1/ 4 + k −1/ 4 )2 x .

Example 6.37 Show that the locus of the middle point of all tangents from points on the directrix to the parabola y 2 = 4 ax is y2(2x + a) = a(x + 3a)2.

Solution Let (-a, y1) be a point on the directrix. Let t be the point of contact of tangents from (-a, y1) to the parabola y 2 = 4 ax . The equation of the tangent at t is yt = x + at 2



Since this passes through (-a, y1), y1 = − a + at t

(6.67) 2

2 \ The point on the directrix is  − a, − a + at  .   t

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Analytical Geometry

Let (x1, y1) be the midpoint of the portion of tangent between the directrix and the point of contact. Then 2 2at + − a + at 2 t x1 = at − a , y1 = 2 2 2 x1 + a = t 2 a



(6.68)

and 2 y1t = 2at 2 − a + at 2 2 y1t = 3at 2 − a ⇒ 4 y12 t 2 = a 2 (3t 2 − 1)2

(i.e.)

(6.69)

2



 3(2 x1 + a)   2x + a  = a2  − 1 using equation (6.68) ∴ 4 y12  1  a  a    ∴ 4 y12 (2 x1 + a) = a[6 x1 + 2a]2



y12 (2 x1 + a) = a[3 x1 + a]2 The locus of (x1, y1) is y 2 (2 x + a) = a(3 x + a)2.

Example 6.38 Tangents are drawn from a variable point P to the parabola y 2 = 4 ax, such that they form a triangle of constant area c2 with the tangent at the vertex. Show that the locus of P is ( y 2 − 4 ax ) x 2 = 4c 4 .

Solution t1 Q P

A R

t2

Let P(x1, y1) be the point of intersection of tangents at t1 and t2. The equation of the tangent at t1 is yt1 = x + at12. This meets the tangent at the vertex at Q. \Q is (0, at1). Similarly, R is (0, at2). P is the point of intersection of tangents at t1 and t2and the point is P(at1t2, a(t1 + t2)). The area of DPQR is given as c2.

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Parabola

1 [0 + 0 + at t ( at − at )] = c 2 1 2 1 2 2



⇒ a 2 t1t2 (t1 − t2 ) = 2c 2

(i.e.) a 4 t12 t22 (t1 − t2 )2 = 4c 4



⇒ a 4 t12 t22 (t1 + t2 )2 − 4t1t2  = 4c 4



⇒ x12 ( y12 − 4 ax1 ) = 4c 4

Therefore, the locus of (x1, y1) is x 2 ( y 2 − 4 ax ) = 4c 4.

Example 6.39 Prove that the distance of the focus from the intersection of two tangents to a parabola is a mean proportional to the focal radii of the point of constant.

Solution Q

P

S

R

Let the tangents at Q( at12 , 2at1 ) and R( at22 , 2at2 ) intersect at P. Then the coordinates of the point P are (at1t2, a(t1 + t2)). S is the point (a, 0).

SQ 2 = ( a − at12 )2 + (2at1 − 0)2 = a 2 (1 − t12 )2 + 4 a 2 t12 = a2 (1 + t12 )2 ∴ SQ = a(1 + t12 ) . Similarly, SR = a(1 + t22 )



SP 2 = ( a − at1t2 )2 + [0 − a(t1 + t2 )]2 ⇒ a 2 (1 − t1t2 )2 + a 2 (t1 + t2 )2 = a 2 (t1 + t2 )2 + (1 − t1t2 )2  = a 2 t22 + t12 + 1 + t12 t22  = a 2 (1 + t12 )((1 + t22 ) \SQ · SR = SP2 \SP is the mean proportional between SQ and SR.

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Analytical Geometry

Example 6.40 Prove that the locus of the point of intersection of normals at the ends of a focal chord of a parabola is another parabola whose latus rectum is one fourth of that of the given parabola.

Solution Let the equation of the parabola be y 2 = 4 ax



(6.70)

Let t1 and t2 bethe ends of a focal chord of the parabola. For a focal chord t1t2 = -1. The equation of the normal at t1 and t2 are

y + xt1 = 2at1 + at13

(6.71)



y + xt2 = 2at2 + at23

(6.72)

Subtracting, x(t1 − t2 ) = 2a(t1 − t2 ) + a(t13 − t23 ) x = 2a + a(t12 + t1t2 + t22 )



From (6.71), y + 2at1 + at1 (t12 + t1t2 + t22 ) = 2at1 + at13 y = − at1t2 (t1 + t2 ) = a(t1 + t2 ) Since t1t2 = −1.



If (x1, y1) is a point of intersection of the normals at t1 and t2 then x1 = 2a + a(t12 + t22 + t1t2 ) = 2a + a[(t1 + t2 )2 + 1] y1 = a(t1 + t2 )

 y2  ( y 2 + a2 ) ∴ x1 = 2a + a  12 + 1 ⇒ x1 − 2a = a 1 2 a a  2 2 or y12 = a1 ( x1 − 3a) ⇒ a( x1 − 2a) = y1 + a

The locus of (x1, y1) is y2 = a(x - 3a) which is a parabola whose latus rectum is one fourth of the latus rectum of the original parabola.

Example 6.41 If the normal at the point t1 on the parabola y2 = 4ax meets the curve again at t2 prove that t2 = −t1 − 2 . t1

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Solution The equation of the normal at t1 is y + xt1 = 2at1 + at13



(6.73)

t1

t2

The equation of the chord joining the points t1 and t2 is y(t1 + t2 ) = 2 x + 2at1t2



(6.74)

Equations(6.73) and (6.74) represent the same lines. Therefore identifying t +t 2at1t2 we get 1 2 = −2 = 1 t1 2at1 + at13 ∴ t1 + t2 = −2 ⇒ t2 = −t1 − 2 t1 t1



Example 6.42 If the normals at two points t1, t2 on the parabola y2 = 4ax intersect again at a point on the curve show that t1 + t2 + t3 = 0 and t1t2 = 2 and the product of ordinates of the two points is 8a2.

Solution The normals t1 and t2 meet at t3. ∴ t3 = −t1 − 2 t1 t3 = −t 2 − 2 t2



Subtracting 0 = −(t1 − t2 ) + 2

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(6.75) (6.76)

(t1 − t2 ) . Since t1 − t2 ≠ 0 , t1t2 = 2. t1t2

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Analytical Geometry

Solving equations (6.75) and (6.76), we get   2t3 = −(t1 + t2 ) − 2  1 + 1   t1 t2 



= −(t1 + t2 ) − 2



\ t1 + t2 + t3 = 0

(t1 + t2 ) = −(t1 + t2 ) − (t1 + t2 ) t1t2

Example 6.43 Find the condition that the line lx + my + n = 0 is a normal to the parabola is y2 = 4ax.

Solution Let the line lx + my + n = 0 be a normal at ‘t’. The parabola is y2 = 4ax. The equation of the normal at t is y + xt = 2at + at 3



(6.77)

But the equation of the normal is given as lx + my = -n



(6.78)

3 Identifying equations (6.77) and (6.78), we get t = 1 = 2at + at ∴ t = l l m −n m  2a + al 2  l = − n 2   and (2a + at )l = 2  m2 



(i.e.)  al 3 + 2alm2 + m2 n = 0 .

Example 6.44 Show that the locus of poles of normal chords of the parabola is y2 = 4ax is ( x + 2a) y 2 + 4 a 3 = 0 .

Solution Let (x1, y1) be the pole of a normal chord normal at t. The equation of the polar of (x1, y1) is

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yy1 = 2a( x + x1 )

(6.79)

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Parabola

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The equation of the normal at t is y + xt = 2at + at 3



(6.80)

Equations (6.79) and (6.80) represent the same line. \ Identifying equations (6.79) and (6.80), we get

y1 −2a 2ax1 = = t t 2at + at 3



t = −2a ; also y1 (2t + t 3 ) = 2 x1 y1

3  y1  −4 a − 8a3  = 2 x1 y1   y1

3

∴ 4 a + 8a2 = −2 x1 y1 3

x1 + 2a + 4 a2 = 0. y1

(i.e.)

The locus of (x1, y1) is ( x + 2a) y 2 + 4 a3 = 0 .

Example 6.45 In the parabola y2 = 4ax the tangent at the point P whose abscissa is equal to the latus rectum meets the axis on T and the normal at P cuts the curve again in Q. Prove that PT:TQ = 4:5.

Solution Let P and Q be the points t1 and t2 respectively. Given that at12 = 4 a or t12 = 4. The equation of the tangent at t1 is

yt1 = x + at12

(6.81)

when this meets the x-axis, y = 0.

∴ x = − at12 = −4 a. Hence T is the point (-4a, 0). Also as the normal at t1 meets the curve at t2,



( t 2 + 2) t2 = −t1 − 2 = − 1 t1 t1



t2 = −6 t1

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(6.82)

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Analytical Geometry

Now, PT 2 = ( at12 + 4 a)2 + (2at1 − 0)2 = 64 a 2 + 16a 2 = 80 a 2 PQ 2 = ( at12 − at22 )2 + (2at1 − 2at2 )2 = ( 4 a − 9a)2 + (2at1 − 2at2 )2

  = 25a2 +  4aat1 + 4 a  = 25a 2 + 64 a 2 + 4 a 2 + 32a 2 t1  



= 125a2

2

∴ PT : PQ = 80 a 2 : 125a 2 = 4 : 5

Example 6.46 Show that the locus of a point such that two of the three normal drawn from it to the parabola y2 = 4ax coincide is 27ay 2 = 4( x − 2a)3 .

Solution Let (x1, y1) be a given point and t be foot of the normal from (x1, y1) to the p­ arabola y2 = 4ax. The equation of the normal at ‘t’ is y + xt = 2at + at 3



(6.83)

Since this passes through (x1, y1) we have

y1 + x1 = 2at + at 3 or at 3 + t (2a − x1 ) − y1 = 0

(6.84)

If t1, t2 and t3 be feet of the normals from (x1, y1) to the parabola then t1, t2 and t3 are the roots of equation (6.84). ∴ t1 + t2 + t3 = 0



t1t2 + t2 t3 + t3t1 =



t1t2 t3 =

(6.85)

2a − x1 a

(6.86)

y1 a

(6.87)

If two of the three normals coincide then t1 = t2.

(6.86) ⇒ 2t1 − t3 = 0 or



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t3 = 2t1 t12 + 2t3t1 =



2a − x1 a

(6.88) (6.89)

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Parabola

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y1 a

(6.90)

From equations (6.88) and (6.89), −2t13 =

y1 −y or t13 = 1 . Since t1 is a root a 2a

t12 t3 =



of equation (6.84) at13 − (2a − x1 )t1 − y1 = 0 .

 −y  a  1  + (2a − x1 )t1 − y1 = 0  2a 





−3 y1 27 y13 = −t1 (2a − x1 ) ⇒ = t13 (2a − x1 )3 2 8



⇒ 27ay13 − 4( x1 − 2a)3 = 0 ⇒ Locus of (x1, y1) is 27ay 2 = 4( x1 − 2a)3.

Example 6.47 If the normals from a point to the parabola y2 = 4ax cut the axis in points whose distances from the vertex are in AP then show that the point lies on the curve 27ay 2 = 2( x − 2a)3.

Solution Let (x1, y1) be a given point and t be the foot of a normal from (x1, y1). The equation of the normal at t is y + xt = 2at + at 3



(6.91)

Since this passes through (x1, y1), y1 + x1t = 2at + at 3. or

at 3 + t (2a − x1 ) − y1 = 0

(6.92)

If t1, t2 and t3 be the feet of the normals from (x1, y1) then t1 + t2 + t3 = 0



t1t2 + t2 t3 + t3t 4 =

2a − x1 y1

(6.93) (6.94)

y1 (6.95) a When the normal at t meets the x-axis, y = 0, from (6.91) we get xt = 2at + at 3 or x = 2a + at 2 .



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t1t2 t3 =

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Analytical Geometry

Then the x-coordinates of the points where the normal meets the x-axis are given by (2a + at12 , 2a + at22 , 2a + at32 ) . Given these are in AP. ∴t12 , t22 and t32 are in AP. 2t22 = t12 + t32

(i.e.)

(6.96)

2t22 = (t1 + t3 )2 − 2t1t3 = t22 − 2t1t3 or 2t1t3 = −t22



From equation (6.95), or t23 =

−2 y1 a

Since t2 is a root of equation (6.91), at23 + t2 (2a − x1 ) − y1 = 0 .  −2 y1  − y1 = −t2 (2a − x1 ) or 3y1 = t2 (2a − x1 ), a  a 



27 y13 = t23 (2a − x1 )3 ⇒ 27ay13 = 2( x1 − 2a)3



\ The locus of (x1, y1) is 27ay 3 = 2( x − 2a)3 .

Example 6.48 Show that the locus of the point of intersection of two normals to the parabola which are at right angles is y 2 = a( x − 3a).

Solution If (x1, y1) is the point of intersection of two normals to the parabola y2 = 4ax then at3 + t(2a - x1) - y1 = 0 (6.97) If t1, t2 and t3 be the feet of the three normals from (x1, y1) then t1 + t2 + t3 = 0 2a − x1 t1t2 + t2 t3 + t3t1 = a y t1t2 t3 = 1 a Since two of the normals are perpendicular then t1t2 = -1



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\t3 =

− y1 a

(6.98) (6.99) (6.100)

(6.101)

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Parabola

6-55

Since t3 is a root of equation (6.97), at33 + (2a − x1 )t3 − y1 = 0. 3

 −y   −y  a  1  − (2a − x1 )  1  − y1 = 0.  a   a 



or − y13 − ay1 (2a − x1 ) − a 2 y1 = 0 or y12 = a( x1 − 3a) The locus of (x1, y1) is y2 = a(x - 3a).

Example 6.49 Prove that a normal chord of a parabola which subtends a right angle at the vertex makes an angle tan −1 ( 2 ) with the x-axis.

Solution Let the equation of the parabola be y2 = 4ax



(6.102)

The equation of the normal at t is P

t

A

Q

y + xt = 2at + t 3



(6.103)

The combined equation of the lines AP and AQ is y2 =



4 ax( y + xt ) or y 2 (2at + at 3 ) = 4 ax( y + xt ) 2at + at 3 4 atx 2 + 4 axy − (2at + at 3 ) y 2 = 0.



Since the two lines are at right angles, coefficient of x2 + coefficient of y = 0. 2



∴ 4 at − 2at − at 3 = 0

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⇒ at (2 − t 2 ) = 0

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6-56

Analytical Geometry

∴ t = 0 or t 2 = 2.



t = 0 corresponds to the normal joining through the vertex. t 2 = 2 ⇒ ( −t ) 2 = 2



tan 2q = 2

or

⇒ q = tan −1 ( ± 2 ) \The normals make an angle tan −1 ( 2 ) with the x-axis.

Example 6.50 Prove that the area of the triangle formed by the normals to the parabola y2 = 4ax at the points t1, t2 and t3is 1 a 2 (t1 − t2 )(t2 − t3 )(t3 − t1 )(t1 + t2 + t3 )2 . 2

Solution The equations of the normals at t1, t2, t3 are

y + xt1 = 2at1 + at13

(6.104)



y + xt2 = 2at2 + at23

(6.105)



y + xt3 = 2at3 + at33

(6.106)

Solving these equations pair wise we get the vertices of the triangle. Hence the vertices are [2a + a(t12 + t1t2 + t22 ), − at1t2 (t1 + t2 )] and two other similar points. 2a + a(t12 + t1t2 + t22 ) − at1t2 (t1 + t2 ) 1 \The area of the triangle = ∆ = 1 2a + a(t22 + t2 t3 + t32 ) − at2 t3 (t2 + t3 ) 1 2 2a + a(t32 + t3t1 + t12 ) − at3t1 (t3 + t1 ) 1

t12 + t1t2 + t22 ∆ = a t22 + t2 t3 + t32 2 2 t3 + t3t1 + t12 2

−t1t2 (t1 + t2 ) 1 − t 2 t 3 (t 2 + t 3 ) 1 −t3t1 (t3 + t1 ) 1



1 t12 + t1t2 + t22 t1t2 (t1 + t2 ) a (t − t )(t + t + t ) −t2 (t3 − t1 )(t1 + t2 + t3 ) 0 = 2 3 1 1 2 3 (t1 − t2 )(t1 + t2 + t3 ) −t3 (t1 − t2 )(t1 + t2 + t3 ) 0



2 = a (t1 + t2 + t3 )2 (t1 − t2 )(t2 − t3 )(t3 − t1 ) 2

2

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Parabola

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Example 6.51 Prove that the length of the intercepts on the normal at the point P(at2, 2at) to the parabola y2 = 4ax made by the circle described on the line joining the focus and P as diameter is a 1 + t 2 .

Solution The equation of the normal at P is y + xt = 2at + at3. P

′t ′

R S

T

Let the circle on PS as diameter cut the normal at P at R and the x-axis at T. SP = ( at 2 − a)2 + (2at )2 = a(1 + t 2 ) 3 SR = at + at = at 1 + t 2 2 1+ t



PR = SP 2 − SR 2 = a 2 (1 + t 2 )2 − a 2 t 2 (1 + t 2 ) = a 2 (1 + t 2 )(1 + t 2 − t 2 ) PR = a 1 + t 2

Example 6.52 Normals at three points P, Q and R of the parabola y2 = 4ax meet in (h, k). 2( h − 2a) from Prove that the centroid of DPQR lies on the axis at a distance 3 the vertex.

Solution Let t be a foot of a normal from (h, k). The equation of the normal at t is y + xt = 2at + at3 (6.107) This passes through (h, k).

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Analytical Geometry



\k + ht = 2at + at3 (6.108) ⇒ at + t (2a - h) - k = 0. If t1, t2 and t3 are the feet of the normals from (h, k) then t1 + t2 + t3 = 0, Σt1t2 = 2a − h , t1t2 t3 = k . a a  at 2 + at22 + at32 2at1 + 2at2 + 2at3  The centroid of the DPQR is  1 ,  3 3  3

Since the centroid lies on the x-axis, t +t +t  2a  1 2 3  = 0 3  



∴ t1 + t2 + t3 = 0 The x-coordinates of the centroid is  t2 + t2 + t2  a  1 2 3  = a [(t1 + t2 + t3 )2 − 2Σt1t2 ] 3   3 = − 2a 2a − h = 2 ( h − 2a) 3 3 a 2 \Centroid is at a distance ( h − 2a) from the vertex of the parabola. 3

(

)

Example 6.53 The normals at three points P, Q and R on a parabola meet at T and S be the focus of the parabola. Prove that SP·SQ·SR = aTS 2.

Solution Let T be the point (h, k). Then P, Q and R are the feet of the normals from T(h, k). The equation of the normal at t is y + xt = 2at + at3. If t1, t2 and t3 be the feet of the normals from T then t1 + t2 + t3 = 0. Σt1t2 = 2a − h , t1t2 t3 = k a a

S is the point (a, 0).

SP 2 = ( a − at12 )2 + (2at1 )2 = a 2 (1 + t12 )2

SP = a(1 + t12 ) SQ = a(1 + t22 ) SR = a(1 + t32 )

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Parabola

6-59

∴ SP ⋅ SQ ⋅ SR = a3 (1 + t12 )(1 + t22 )(1 + t32 ) = a3 [1 + Σt12 + Σt12 t22 + (t1t2 t3 )2 ] = a3 [1 + (t1 + t2 + t3 )2 − 2Σt1t2 + (t1t2 + t2 t3 + t3t1 )2 − 2t1t2 t3 (t1 + t2 + t3 ) + (t1t2 t3 )2 ]



 2(2a − h) (2a − h)2 k 3  = a3 1 − + + 3 a a2 a   = a  a 2 − 2a(2a − h) + (2a − h)2 + k 3  = a  a 2 − 2ah + h2 + k 2 



= a ( h − a)2 + k 2  = a ⋅ ST 2

Example 6.54 The equation of a chord PQ of the parabola y2 = 4ax is lx + my = 1. Show that 2 , 4 am  . the normals at P, Q meet on the normal at  4 am 2  l l 

Solution Let P and Q be the points t1 and t2. The normals at P and Q meet at R. If t3 is the foot of the normal of the 3rd point then t1 + t2 + t3 = 0 (6.109) The equation of the chord PQ is lx + my = 1. Since P and Q are the points t1 and t2, lat12 + 2amt1 = 1 lat22 + 2amt2 = 1

\la(t1 - t2)(t1 + t2) + 2am(t1 - t2) = 0 Since t1 - t2 ≠ 0, la + 2am(-t3) = 0 or t3 = 2m n 2 Hence, the 3rd point ( at32 , 2at3 ) is  4 am , 4 am  . 2  l l 

Example 6.55 If the normal at P to the parabola y2 = 4ax meets the curve at Q and make an angle q with the axis show that (i) it will cut the parabola at q  at an angle tan −1 1 tan q and 2 (ii) PQ = 4a secq  cosec2q.

(

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)

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Analytical Geometry

Solution Let P be the point (at 2, 2at). The equation of the normal at t is y + xt = 2at + at3. The normal at t meets the curve at Q( at12 , 2at1 ). ∴ t1 = −t − 2 t



Let f  be the angle between the normal and the tangent at Q. The slope of the tangent at Q is 1 = 1 = 2−t t1 −t − 2 t + 2 t Slope of the normal at t is -t. t −t (t 2 + 2) t2 + 2 = ∴ tan f = = t 2 2 2 2 2 t ( t + ) 1+ 2 t +2 = 1 tan q since tan q = − t 2 −t +



Since tanq = -t

(

∴ f = tan −1 1 tan q 2



)

PQ 2 = ( at 2 − at12 )2 + (2at − 2at1 )2 = a2 (t + t1 )2 (t − t1 )2 + 4 a2 (t − t1 )2



(

2  = a 2 (t − t1 )2 [(t1 + t )2 + 4] = a 2 t + t + 2   t − t − 2   t  t

= a2 =

4(t 2 + 1)2 t2

) + 4 2

2 2 2 2  4 + 4 = 16a (1 + t ) (1 + t )  2  4 t t

16a2 (1 + t 2 )3 16a2 sec6 q = 16a 2 sec2 q cosec 4q = tan 4 q t4

⇒ PQ = 4 a secq cosec2q

Example 6.56 Prove that the circle passing through the feet of the three normals to a ­parabola drawn from any point in the plane passes through the vertex of the

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Parabola

6-61

parabola. Also find the equation of the circle passing through the feet of the normals.

Solution Let the equation of the parabola be y2 = 4ax



(6.110)

Let the equation of the circle be

x2 + y2 + 2gx + 2fy + c = 0

(6.111)

Let P, Q and R be the feet of the normals to y2 = 4ax from a given point (h, k). Then we have at3 + (2a - h)t - k = 0. If t1, t2 and t3 be the feet of the normals at P, Q and R then t1 + t2 + t3 = 0. We know that the circle (6.111) and the parabola (6.110) cut at four points and if t1, t2, t3 and t4 are the four points of intersection of the circle and the parabola then they are the roots of the equation,

a2t4 + (4a2 + 2ga)t2 + 4fat + c = 0

(6.112)



\t1 + t2 + t3 + t4 = 0

(6.113)

If t1, t2 and t3 correspond to the feet of the normals from (h, k) then

t1 + t2 + t3 = 0

(6.114)

From equations (6.113) and (6.114), t4 = 1. But t4 is the point ( at 42 , 2at 4 ) (i.e.) (0, 0) which is the vertex of the ­parabola. Hence the circle passing through the feet of the normals from a ­given point also passes through the vertex of the parabola. Hence equation (6.112) becomes at3 + (4a + 2g)t + 4f = 0 (6.115) since c = 0 Equations (6.111) and (6.115) are the same. By comparing the coefficients, 4a + 2 g 4 f we get 1 = = −k 2a − h

\ 2a - h = 4a + 2g and 4f = -k or -2a - h = 2g and 2 f = − k 2 \ The equation of the circle passing through the feet of the normal is x 2 + y 2 − (2a + h) x − k y = 0 or 2(x2 + y2) - 2(2a + h) x - ky = 0. 2

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Analytical Geometry

Exercises 1.

Show that the portion of the tangent intercepted between the point of contact and the directrix subtends a right angle at the focus.

2.

If the tangent at a point P on the parabola meets the axis at T and PN is the ordinate at P then show that AN = AT.

3.

If the tangent at P meets the tangent at the vertex in Y then show that SY is perpendicular to TP and SY 2 = AS SP.

4.

If A, B and C, are three points on a parabola whose ordinates are in GP then prove that the tangents at A and C meet on the ordinates of B.

5.

Prove that the middle point of the intercepts made on a tangent to a parabola by the tangents at two points P and Q lies on the tangent which is parallel to PQ.

6.

If points (at2, 2at) is one extremity of a focal chord of the parabola y2 = 4ax, 2  1 show that the length of the focal chord is a  t +  .  t

7.

Show that the tangents at one extremity of a focal chord of a parabola is parallel to the normal at the other extremity.

8.

If the tangents at three points on the parabola y2 = 4ax make angles 60°, 45° and 30° with the axis of the parabola, show that the abscissae and ordinates of the three points are in GP.

9.

Show that the circle described on the focal chord of a parabola as diameter touches the directrix.

10.

Show that the tangent at one extremity of a focal chord of a parabola is parallel to the normal at the other extremity.

11.

Prove that the semilatus rectum of a parabola is the harmonic mean of the segments of a focal chord.

12.

Prove that the circle described on focal radii as diameter touches the tangents at the vertex of a parabola.

13.

Three normals to a parabola y2 = 4x are drawn through the point (15, 12). Show that the equations are 3x - y - 33 = 0, 4x + y - 72 = 0 and x - y - 3 = 0.

14.

The normals at two points P and Q of a parabola y2 = 4ax meet at the point (x1, y1) on the parabola. Show that PQ = (x1 + 4a) (x1 - 8a).

15.

Show that the coordinates of the feet of the normals of the parabola y2 = 4ax drawn from the point (6a, 0) are (0, 0), (4a, 4a) and (4a, -4a).

16.

The normal at P to the parabola y2 = 4ax makes an angle a  with the axis. Show that the area of the triangle, formed by it is the tangents at its extremities is a constant.

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Parabola

17.

If P, Q and R are the points t1, t2 and t3 on the parabola y2 = 4ax, such that the normal at Q and R meet at P then show that: (i) the line PQ is passes through a fixed point on the axis. (ii) the locus of the pole of PQ is x = a. (iii) the locus of the midpoint of PQ is y2 = 2a(x + 2a). (iv) the ordinates of P and Q are the roots of the equation y2 + xy + 8a2 = 0 where t3 is the ordinate of the point of intersection of the normals at P and Q.

18.

If a circle cuts a parabola at P, Q, R and S show that PQ and RS are equally inclined to the axis.

19.

The normals at the points P and R on the parabola y2 = 4ax meet on the ­parabola at the point P. Show that the locus of the orthocentre of DPQR is y2 = a(x + 6a) and the locus of the circumcentre of DPQR is the parabola 2y = x(x - a).

20.

Prove that the area of the triangle inscribed in a parabola is twice the area of the triangle formed by the tangents at the vertices.

21.

Prove that any three tangents to a parabola whose slopes are in HP encloses a triangle of constant area.

22.

Prove that the circumcircle of a triangle circumscribing a parabola passes through the focus.

23.

If the normals at any point P of the parabola y2 = 4ax meet the axis at G and the tangent at vertex at H and if A be the vertex of the parabola and the rectangle AGQH be completed, prove that the equation to the locus of Q is x2 = 2ax + ay2.

24.

The normal at a point P of a parabola meets the curve again at Q and T is the pole of PQ. Show that T lies on the directrix passing through P and that PT is bisected by the directrix.

25.

If from the vertex of the parabola y2 = 4ax, a pair of chords be drawn at right angles to one another and with these chords as adjacent sides a rectangle be made then show that the locus of further angle of the rectangle is the parabola y2 = 4a(x - 8a).

26.

The normal to the parabola y2 = 4ax at a point P on it meets the axis in G. Show that P and G are equidistant from the focus of the parabola.

27.

Two perpendicular straight lines through the focus of the parabola y2 = 4ax meet its directrix in T and T ′ respectively. Show that the tangents to the parabola to the perpendicular lines intersect at the midpoint of TT ′.

28.

If the normals at any point P(18, 12) to the parabola y2 = 8x cuts the curve again at Q show that 9 ⋅ PQ = 80 10 .

29.

If the normal at P to the parabola y2 = 4ax meets the curve again at Q and if PQ and the normal at Q make angles q and f, respectively with the axis, prove that tanq tan f + tan 2 q + 2 = 0.

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Analytical Geometry

30.

PQ is a focal chord of a parabola. PP′ and QQ′ are the normals at P and Q cutting the curve again at P′ and Q′. Show that P′Q′ is parallel to PQ and is three times PQ.

31.

If PQ be a normal chord of the parabola. y2 = 4ax and if S be the focus, show that the locus of the centroid of the triangle SPQ is y2(ay2 + 180a2 - 108ax) + 128 a4 = 0.

32.

If the tangents at P and Q meet at T and the orthocenter of the ∆PTQ lies on the parabola, show that either the orthocentre is at the vertex or the chord PQ is normal to the parabola.

33.

If three normals from a point to the parabola y2 = 4ax cuts the axis in points, whose distances from the vertex are in AP, show that the point on the curve 27ay 2 = 2( x − a)3 .

34.

Tangents are drawn to a parabola from any point on the directrix. Show that the normals at the points of contact are perpendicular to each other and that they intersect on another parabola.

35.

Show that if two tangents to a parabola y2 = 4ax intercept a constant length on any fixed tangent, the locus of their point of intersection is another equal parabola.

36.

Show that the equation of the circle described on the chord intercepted by the parabola y2 = 4ax on the line y = mx + c as diameter is m2(x2 + y2) + 2(mc - 2a) x - 4ay + c(4am + c) = 0.

37.

Circles are described on any two common chords of a parabola as diameter. Prove that their common chord passes through the vertex of the parabola.

38.

If P(h, k) is a fixed point in the plane of a parabola y2 = 4ax. Through P a variable secant is drawn to cut the parabola in Q and R. T is a point on QR such that (i) PQ · PR = PT 2. Show that the locus of T is (y - k)2 = k2 - 4ah. (ii) PQ + PR = PT. Show that the locus of T is y2- k2 = 4a (x - h).

39.

40. 41.

Show that the locus of the point of intersection of tangents, to the parabola 2 a b  y2 = 4ax at points whose ordinates are in the ratio a 2:b 2 is y 2 =  +  ax. b a Show that the locus of the middle points of a system of parallel chords of a parabola is a line which is parallel to the axis of the parabola. P, Q and R are three points on a parabola and the chord PQ meets the diameter through R in T. Ordinates PM and QN are drawn to this diameter. Show that RMRN = RT 2.

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Chapter

7

Ellipse

7.1  STANDARD EQUATION A conic is defined as the locus of a point such that its distance from a fixed point bears a constant ratio to its distance from a fixed line. The fixed point is called the focus and the fixed straight line is called the directrix. The constant ratio is called the eccentricity of the conic. If the eccentricity is less than unity the conic is called an ellipse. Let us now derive the standard equation of an ellipse using the above property called focus-directrix property.

7.2  STANDARD EQUATION OF AN ELLIPSE y

l L

X′

A′

S′

C

S

P(x, y)

N

A

M x X

L′

Let S be the focus and line l be the directrix. Draw SX perpendicular to the directrix. Divide SX internally and externally in the ratio e:1(e < 1). Let A and SA′ SA A′ be the points of division. Since = e, from the definition = e and A′ X AX of ellipse, the points A and A′ lie on the ellipse. Let AA′ = 2a and C be its middle point.

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SA = eAX

(7.1)

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7-2

Analytical Geometry

SA′ = eA′X





(7.2)

Adding equations (7.1) and (7.2), we get SA + SA′ = e(AX + A′X).

(i.e.)  AA′ = e(AX + A′X) = e(CX - CA + CX + CA′) = e ⋅ 2CX Since CA = CA′



a ∴ CX = e



(7.3)

Subtracting equations (7.1) from (7.2), we get SA′ - SA = e(CX′ − CX)

CS + CA + (CS − CA) = e(AA′)



2CS = e ⋅ 2a ⇒ CS = ae

(7.4)

Take CS as the x-axis and CM perpendicular to CS, as y-axis. Let P(x, y) be any point on the ellipse. Draw PM perpendicular to the directrix. Then the coordinates of S are (ae, 0). From the focus-directrix propSP = e. erty of the ellipse, PM ∴ SP2 = e2 PM 2 = e2 NX 2 = e2(CX − CN )2

a  (i.e.)  (x − ae)2 + y2 = e2 (CX − CN)2 = e 2  − x  e 



x 2 − 2aex + a 2 e 2 + y 2 = a 2 − 2aex + e 2 x 2



x2(1 - e2) + y2 = a2(1 - e2)

2

Dividing by a2(1 − e2), we get

x2 y2 + 2 =1 2 a a (1 − e 2 )



(i.e.) 

x2 y2 + = 1 a2 b2

(7.5)

where  b2 = a2(1 - e2) This is called the standard equation of an ellipse.

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7-3

Ellipse

Note 7.2.1:   1. Equation (7.5) can be written as:

y2 x 2 a 2 − x 2 ( a + x )( a − x ) = 1− 2 = = 2 b a a2 a2



PN 2 AN ⋅ NX ′ PN 2 b 2 BC 2 = ⇒ = = AN ⋅ NA′ a 2 AC 2 b2 a2

  2.   3.   4.   5.   6.   7.

AA′ is called the major axis of the ellipse. BB′ is called the minor axis of the ellipse. C is called the centre of the ellipse. The curve meets the x-axis at the point A(a, 0) and A′(−a, 0). The curve meets the y-axis at the points B(0, b) and B′(0, −b). The curve is symmetrical about both the axes. If (x, y) is a point on the curve, then (x, −y) and (−x, y) are also the points on the curve.   8. From the equation of the ellipse, we get

x=±

a b2 − y 2 b a2 − x 2 ,y=± a b

Therefore, for any point (x, y) on the curve, −a ≤ x ≤ a and −b ≤ y ≤ b.   9. The double ordinate through the focus is called the latus rectum of the ellipse. (i.e.)  LSL′ is the latus rectum.

a  Length of latus rectum = LL ′ = 2SL = 2e (CX − CS ) = 2e  − ae e 



= 2a(1 − e2).



= 2a ⋅

b 2 2b 2 = a a2

10. Second focus and second directrix: On the negative side of the origin, take a point S′ such that CS = CS′ and another point X′ such that CX = CX ′ = a. Draw X′M′ perpendicular to AA′ and PM′ perpendicular to X′M′. S′P x2 y2 = e gives the locus of P as 2 + 2 = 1. Then we can show that PM ′ a b

Here S′is called the second focus and X′M′ is the second directrix.

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Analytical Geometry

11. (i) Shifting the origin to the focus S, the equation of the ellipse ( x − ae)2 y 2 + 2 = 1. is a2 b (ii) Shifting the origin to A, the equation of the ellipse is ( x − a) 2 y 2 + 2 = 1. a2 b (iii) Shifting the origin to X, the equation of the focus is 2 a   x −  2 2 + y =1 . a2 b2 12. The equation of an ellipse is easily determined if we are given the focus and the equation of the directrix.

7.3  FOCAL DISTANCE The sum of the focal distances of any point on the ellipse is equal to the length of the major axis. In the above figure, (section 2.2)

SP = ePM



S′P = ePM 2.

Adding

SP + S′P = e(PM + PM′) = e(NX + NX′)



a a  = e[CX − CN + CX′ + CN] = e  − x1 + + x1  e e  = 2a where P is (x1, y1).



= length of the major axis



Note 7.3.1:

SP = ePM = a - ex1



S′P = ePM′ = a + ex1

7.4  POSITION OF A POINT A point (x1, y1) lies inside, on or outside of the ellipse according as x 2 y2 + - 1 is negative, zero or positive. a 2 b2

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Ellipse

7-5

Let Q(x1, y1) be a point on the ordinate PN where P is a point on the ellipse x2 y2 + − 1 = 0. a2 b2 Then, QN < PN ⇒

QN 2 PN 2 y′2 x′2 < ⇒ < 1 − b2 b2 b2 a2 ∴



x′2 y′2 + 2 1. a b x′2 y′2 Evidently if Q(x′, y′) is a point on the ellipse, 2 + 2 − 1 = 0. a b

7.5  AUXILIARY CIRCLE The circle described on the major axis as diameter is called the auxiliary circle. Let P be any point on the ellipse. Let the ordinate through P meet the ­auxiliary circle at P′. Since A′ P ′ A = 90°, we have the geometrical relation, P′N2 = AN·A′N. y

P′

P A′

O

N

A

x

However, we know that PN 2 = AN·A′N. ∴ PN 2 : P′N 2 = b2 : a2 or

PN b = P ′N a

The point P′ where the ordinate PN meets the auxiliary circle is called the corresponding point of P. Therefore, the ordinate of any point on the ellipse to that of corresponding point on the ellipse are in the ratios of lengths of ­semi-minor

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7-6

Analytical Geometry

axis and semi-major axis. This ratio gives another definition to an ellipse. ­Consider a circle and from each point on it, draw perpendicular to a diameter. The locus of these points dividing these perpendiculars in a given ratio is an ellipse and for this ellipse the given circle is the auxiliary circle.

ILLUSTRATIVE EXAMPLES BASED ON FOCUS-DIRECTRIX property Example 7.1 Find the equation of the ellipse whose foci, directrix and eccentricity are given below: (i) Focus is (1, 2), directrix is 2x − 3y + 6 = 0 and eccentricity is 2 3 (ii) Focus is (0, 0), directrix is 3x + 4y − 1 = 0 and eccentricity is 5 6 (iii) Focus is (1, −2), directrix is 3x − 2y + 1 = 0 and eccentricity is 1 2

Solution (i) Let P ( x1, y1) be a point on the ellipse. Then \ SP2 = e2 PM 2



SP SP 2 =e⇒ = e2 PM PM 2

SP = ( x1 − 1)2 + ( y1 − 2)2 ,



PM = ±

Hence,

2 x1 − 3 y1 + 6 13

( x1 − 1)2 + ( y1 − 2)2 =

2 ;e= . 3

4  2 x1 − 3 y1 + 6   9  13

2

117[ x12 − 2 x1 + 1 + y12 − 4 y1 + 4] = 4[4 x12 + 9 y12 + 36 − 12 x1 y1 + 24 x1 − 36 y1 ] Therefore, the locus of (x1, y1) is the ellipse 101x2 + 81y2 + 48x − 330x − 324y + 441 = 0. 3 x + 4 y1 − 1 5 (ii) SP = x12 + y12 ; PM = 1 ,e= 5 6

SP 2 = e 2 PM 2 ⇒ x12 + y12 =

25  3 x1 + 4 y1 − 1  36  5 

2

⇒ 36( x12 + y12 ) = 9 x12 + 16 y12 + 1 + 24 x1 y1 − 6 x1 − 8 y1

Therefore, the locus of (x1, y1) is the ellipse 27x2 + 20y2 − 24xy + 6x + 8y − 1 = 0.

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7-7

Ellipse

SP = ( x1 − 1)2 + ( y1 + 2)2

(iii)

⇒ SP 2 = x12 + y12 − 2 x1 + 4 y1 + 5 PM = ±



13

; e=

1 2

, SP 2 = e 2 PM 2

1  3 x − 2 y1 + 1 x + y − 2 x1 + 4 y1 + 5 =  1  2 13 2 1



3 x1 − 2 y1 + 1

2

2 1

26( x12 + y12 − 2 x1 + 4 y1 + 5) = 9 x12 + 4 y12 + 1 − 12 x1 y1 + 6 x − 4 y1

Therefore, the locus of (x1, y1) is the ellipse 17x2 + 22y2 + 12xy − 58x + 108y + 129 = 0.

Example 7.2 Find the equation of the ellipse whose (i) Foci are (4, 0) and (−4, 0) and e = (ii) Foci are (3, 0) and (−3, 0) and e =

1 3 3 8

Solution (i) If the foci are (ae, 0) and (−ae, 0) then the equation of the ellipse is x2 y2 1 + 2 = 1. Here, ae = 4 and e = . 2 3 a b

a=

4 = 4 × 3 = 12 e

8  1 b 2 = a 2 (1 − e 2 ) = 144 1 −  = 144 × = 128  9 9

∴ The equation of the ellipse is

x2 y2 + =1. 144 128

x2 y2 (ii) If the foci are (ae, 0) and (−ae, 0) the equation of the ellipse is 2 + 2 = 1. a b 3 3 2 2 Here, ae = 3 and e = ; a e = 9 and e 2 = 8 8

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7-8

Analytical Geometry

∴ a2 = 9 ×



8  5 = 24, b2 = a2 (1 − e 2 ) = 24   = 15  8 3

Therefore, the equation of the ellipse is

x2 y2 + = 1. 24 15

Example 7.3 Find the eccentricity, foci and the length of the latus rectum of the ellipse. (i) 9x2 + 4y2 = 36 (ii) 3x2 + 4y2 − 12x − 8y + 4 = 0 (iii) 25x2 + 9y2 − 150x − 90y + 225 = 0.

Solution (i) 9x2 + 4y2 = 36 Dividing by 36, we get 9x2 4 y2 + =1 36 36





(i.e.) 

∴ a 2 = 4, b 2 = 9.



x2 y2 + =1 4 5

This is an ellipse whose major axis is the y-axis and minor axis is the x-axis and centre at the origin. ∴ a2 = b2 (1 - e2) ⇒ 4 = 9(1 − e2) ∴ 9e2 = 5



Therefore, eccentricity = e =

5 3

be   Therefore, foci are  0, ±  (i.e.) (0, ± 5 ).  1 Therefore, latus rectum = (ii)

2a 2 4 8 =2× = . b 3 3

3x2 + 4y2 − 12x − 8y + 4 = 0 (3x − 12x) + (4y2 − 8y) + 4 = 0 2

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Ellipse

7-9

3(x2 − 4x) + 4(y2 − 2y) + 4 = 0 2 3(x − 4x + 4) − 12 + 4 (y2 − 2y + 1) − 4 + 4 = 0 ⇒3(x − 2)2 + 4( y − 1)2 = 12 ( x − 2)2 ( y − 1)2 + =1 4 3 Shift the origin to the point (2, 1). Therefore, centre is (2, 1). ⇒

y

(2, 1) (1, 1)

(3, 1) x

Therefore, the equation of the ellipse is X2 Y2 + =1 4 3



a2 = 4, b2 = 3



a2e2 = a2 − b2 = 4 − 3 = 1



1 1 1 or e = ∴ ae = 2 × = 1 4 2 2 Therefore, foci are (3, 1) and (1, 1) with respect to old axes.



4e 2 = 1 or e 2 =

Length of the latus rectum =

2b 2 3 = 2 × = 3. a 2

(iii)

25x2 + 9y2 − 150x − 90y + 225 = 0



(25x2 − 150x) + (9y2 − 90y) + 225 = 0



25(x2 − 6x) + 9(y2 − 10y) + 225 = 0

2 5 25[( x − 3) − 9] + 9[( y − 5) − 25] + 225 = 0 25(x − 3)2 + 9(y − 5)2 = 225



⇒

( x − 3)2 ( y − 5)2 + = 1. 9 25

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7-10



Analytical Geometry

Shift the origin to the point (3, 5). X2 Y2 Therefore, the equation of the ellipse is + = 1. 9 25 Therefore, centre is (3, 5). This is an ellipse with y-axis on the major axis and x-axis as the minor axis. Y

y

(3, 10)

(3, 9)

(3, 5)

x

(3, 1) X

(3, 0)

a2 = 9, b2 = 25 a = 3, b = 5 2 2 b e = b2 − a2 25e2 = 25 - 9 = 16



e2 =



16 4 ⇒e= 25 5

Therefore, foci lie on the line x = 3. be = 5 ×



4 =4 5

Therefore, foci are (3, 9) and (3, 1) and Leangth of the latus 2a 2 18 rectum = = . b 5

Exercises 1.

Find the centre, foci and latus rectum of the ellipse:



(i) 3 x 2 + 4 y 2 + 12 x + 8 y − 32 = 0 



(ii) 9 x + 25 y = 225  2

2

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Ans.: (-2, -1); (0, -1); (-4, -1); 6 9 Ans.: (0, 0); (± 7, 0); 8

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Ellipse

7-11

2 3 2 2 (iv) 2 x 2 + 3 y 2 − 4 x + 6 y + 4 = 0  Ans.: (-1, -1); (1 ± 1 , -1); 3 6 2. Find the equation of the ellipse whose foci are (0, ±2) and the length of major axis is 2 5. Ans.: 5x2 + y2 = 5 1 3. Find the equation of the ellipse whose foci is (3, 1), eccentricity and directrix 2 is x - y + 6 = 0.



(iii) x 2 + 9 y 2 = 9 

Ans.: (0, 0); (± 6 , 0);

Ans.: 7x2 + 2xy + 7y2 - 60x - 20y + 44 = 0

 4.  5.

Find the equation of ellipse whose centre is at the origin, one focus is (0, 3) and the length of semi-major axis is 5. x2 y2 + =1 Ans.: 12 25 1 Find the equation of ellipse whose focus is (1, −1), eccentricity is and 2 directrix is x - y + 3 = 0. Ans.: 7x2 + 2xy + 7y2 - 22x + 22y + 7 = 0

 6.  7.  8.  9.



Find the equation of the ellipse whose centre is (2, −3), one focus at (3, −3) and one vertex at (4, −3). Ans.: 3x2 + 4y2 - 12x + 24y + 36 = 0 Find the coordinates of the centre, eccentricity and foci of the ellipse 8x2 + 6y2 6x + 12y + 13 = 0 Ans.: (1, −1),1,( −2, 6 ± 1) Find the equation of the ellipse with foci at (0, 1) and (0, −1) and minor axis of length 1. Ans.: 2x2 + 4y2 = 5 An ellipse is described by using one endless string which is passed through two points. If the axes are 6 and 4 units find the necessary length and the distance between the points. Ans.: 6 + 2 5 , 2 5

7.6  CONDITION FOR TANGENCY To find the condition that the straight line y = mx + c may be a tangent to the ellipse: Let the equation of the ellipse be

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x2 y2 + = 1 a2 b2

(7.6)

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7-12

Analytical Geometry

Let the equation of the straight line be y = mx + c



(7.7)

Solving equations (7.6) and (7.7), we get their points of intersection; the x 2 ( mx + c)2 x-coordinates of the points of intersection are given by 2 + = 1. a b2

(i.e.)  b2x2 + a2(mx + c)2 = a2b2



(i.e.)  x2 (b2 + a2m2) + 2cma2x + a2(c2 − b2) = 0

(7.8)

If y = mx + c is a tangent to the ellipse then the two values of x of this equation are equal. The condition for that is the discriminant of the quadratic equation is zero. ∴ 4a 4 m2 c 2 − 4(a 2 m2 + b 2 ) ⋅ a 2 (c 2 − b 2 ) = 0

a2m2c2 − (a2m2 + b2) (c2 − b2) = 0



a2m2c2 − (a2m2c2 − a2m2b2 + b2c2 - b4) = 0 b2c2 = a2m2b2 + b4

or

∴ c2 = a2m2 + b2



This is the required condition for the line y = mx + c to be a tangent to the given ellipse.

Note 7.6.1: The equation of any tangent to the ellipse is given by y = mx + a 2 m2 + b 2 .

7.7  DIRECTOR CIRCLE OF an ELLIPSE To show that always two tangents can be drawn from a given point to an ellipse and the locus of point of intersection of perpendicular tangents is a circle: Let the equation of the ellipse be x2 y2 + = 1 a2 b2

(7.9)

y = mx + a 2 m2 + b 2

(7.10)

Any tangent to this ellipse is

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Ellipse

7-13

If this tangent passes through the point (x1, y1) then y1 = mx1 + a 2 m2 + b 2 .

(i.e.)  (y1 − mx1)2 = a2m2 + b2 ∴ m2 ( x12 − a 2 ) − 2mx1 y1 + y12 − b 2 = 0

(7.11)

This is a quadratic equation in m and hence there are two values for m. For each value of m, there is a tangent (real or imaginary) and hence there are two tangents from a given point to an ellipse. If m1 and m2 are the roots of the −2 x y y 2 − b2 . equation (7.11), then m1 + m2 = 2 1 12 , m1m2 = 12 x1 − a 2 x1 − a If the two tangents are perpendicular then m1m2 = −1. y12 − b 2 = −1 or y12 − b 2 = − x12 + a 2 x12 − a 2



∴



⇒ x12 + y12 = a 2 + b 2

The locus of (x1, y1) is x2 + y2 = a2 + b2 which is a circle, centre at (0, 0) and radius a 2 + b 2 .

Note 7.7.1: This circle is called the director circle of the ellipse.

7.8  EQUATION OF THE TANGENT To find the equation of the chord joining the points (x1, y1) and (x2, y2) and find the equation of the tangent at (x1, y1) to the ellipse: Let P(x1, y1) and Q(x2, y2) be two points on the ellipse. Let the equation of ellipse be

x2 y2 + = 1 a2 b2

(7.12)

x12 y12 + = 1 a2 b2

(7.13)

x22 y22 + = 1 a2 b2

(7.14)

Then and

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7-14

Analytical Geometry

Subtracting,

x12 − x22 y12 − y12 + =0 a2 b2 ( x1 − x2 )( x1 + x2 ) ( y1 − y2 )( y1 + y2 ) + = 0 a2 b2

(7.15)

From equation (7.15), we get the equation of the chord joining the points (x1, y1) and (x2, y2) as:

y − y1 y1 − y2 −b 2  x1 + x2  = = 2  x − x1 x1 − x2 a  y1 + y2 

(7.16)

This chord becomes the tangent at (x1, y1) if Q tends to P and coincides with P. Hence, by putting x2 = x1 and y2 = y1 in equation (7.16), we get the equation of the tangent at (x1, y1). Therefore, the equation of the tangent at (x1, y1) is:



y − y1 −b 2 x1 = 2 x − x1 a y1 or a 2 yy1 − a 2 y12 = −b 2 xx1 + b 2 x12 b 2 xx1 + a 2 yy1 = b 2 x12 + a 2 y12

Dividing by a2b2, we get

xx1 yy1 x12 y12 + 2 = 2 + 2 a2 b a b

(7.17)

x12 y12 + = 1 since (x1, y1) lies on the ellipse. a2 b2 Therefore, from equation (7.17), the equation of the tangent at (x1, y1) is xx1 yy1 + 2 = 1. a2 b

However,

7.9  EQUATION OF TANGENT AND NORMAL To find the equation of tangent and normal at (x1, y1) to the ellipse x 2 y2 + = 1: a 2 b2 The equation of the ellipse is

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x2 y2 + = 1 a2 b2

(7.18)

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Ellipse

7-15

Differentiating with respect to x, we get



2 x 2 y dy dy −b 2 x 0 + = ⇒ = 2 dx a y a 2 b 2 dx −b2 x  dy  = 2 1   dx (x1, y1 ) a y1

However, dy at (x1, y1) = slope of the tangent at (x1, y1). Therefore, the equadx tion of the tangent at (x1, y1) is, y − y1 =



−b 2 x1 ( x − x1 ) a 2 y1



or a 2 yy1 − a 2 y12 = −b 2 xx1 + b 2 x12



or a 2 yy1 + b 2 xx1 = b 2 x12 + a 2 y12

Dividing by a2b2, we get

xx1 yy1 x12 y12 x2 y2 + 2 = 2 + 2 = 1 since 12 + 12 = 1. 2 a b a b a b

Slope of the normal at (x1, y1) is

a 2 y1 . b 2 x1

The equation of the normal at (x1, y1) to the ellipse is y − y1 =



a 2 y1 ( x − x1 ) b 2 x1

b 2 x1 y − b 2 x1 y1 = a 2 y1 x − a 2 x1 y1 or

b 2 x1 y − a 2 xy1 = (b 2 − a 2 ) x1 y1

Dividing by x1, y1, we get,

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a2 x b2 y − = a2 − b2 x1 y1

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7-16

Analytical Geometry

x2 y2 Therefore, the equation of normal at (x1, y1) to the ellipse 2 + 2 = 1 is a b a2 x b2 y − = a 2 − b 2. x1 y1

7.10  EQUATION TO THE CHORD OF CONTACT To find the equation to the chord of contact of tangents drawn from x 2 y2 (x1, y1) to the ellipse 2 + 2 = 1: a b The equation of the ellipse is x2 y2 + =1 a2 b2



(7.19)

Q (x2, y2)

P (x1, y1) R (x3, y3)

Let QR be the chord of contact of tangents from P(x1, y1). Let Q and R be the points (x2, y2) and (x3, y3), respectively. Then the equation of tangents at Q and R are:

xx2 yy2 + 2 = 1 a2 b

(7.20)



xx3 yy3 + 2 = 1 a2 b

(7.21)

These two tangents pass through P(x1, y1). Therefore,

xx yy x1 x2 y1 y2 + 2 = 1 and 1 2 3 + 1 2 3 = 1. 2 a b a b

The above two equations show that the points (x2, y2) and (x3, y3) lie on the line xx1 yy1 xx yy + 2 = 1. Hence, the equation of the chord of contact is 21 + 21 = 1. 2 a b a b

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Ellipse

7-17

7.11  EQUATION OF THE POLAR To find the equation of the polar of the point P(x1, y1) on the ellipse x 2 y2 + = 1: a 2 b2 , y 1) P (x 1 , k)

Q

T (h

R

Let P(x1, y1) be the given point. Let QR be a variable chord through the point P(x1, y1). Let the tangents at Q and R meet at T(h, k). The equation of the chord contact from T(h, k) is: xh yk + = 1 a2 b2



(7.22)

This chord of contact passes through (x1, y1).

∴

x1h y1k + 2 = 1 a2 b

(7.23)

The locus of T(h, k) is the polar of the point (x1, y1). Therefore, the polar of (x1, y1) is

xx1 yy1 + 2 = 1. a2 b

Note 7.11.1: 1. When the point (x1, y1) lies on the ellipse, the polar of (x1, y1) is the tangent at (x1, y1). When the point (x1, y1) lies inside the ellipse the polar of (x1, y1) is the chord of contact of tangents from (x1, y1). xx yy 2. The line 21 + 21 = 1 is called the polar of the point (x1, y1) and (x1, y1) a b xx yy is called the pole of the line 21 + 21 = 1. a b

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7-18

Analytical Geometry

7.12  CONDITION FOR CONJUGATE LINES To find the pole of the line lx + my + n = 0 with respect to the ellipse x 2 y2 + = 1 and deduce the condition for the lines lx + my + n = 0 and a 2 b2 l1x + m1y + n1 = 0 to be conjugate lines: Let (x1, y1) be the pole of the line

lx + my + n = 0

(7.24)

x2 y2 + =1 a2 b2

(7.25)

xx1 yy1 + 2 = 1 . a2 b

(7.26)

with respect to the ellipse Then the polar of (x1, y1) is:

Then the equations (7.24) and (7.26) represent the same line. ∴ Identifying equations (7.24) and (7.26), we get



x1 /a 2 y1 /b 2 −1 = = l m n 2 −la − mb 2 ∴ x1 = , y1 = n n

 −la 2 − mb 2  , . Two lines are Hence, the pole of the line lx + my + n = 0 is  n   n said to be conjugate if the pole of the each lies on the other.  −la 2 − mb 2  ∴ The point  , lies on the line l1x + m1y + n1 = 0. n   n

 − mb 2   −la 2  + m1  l1  + n1 = 0   n   n 



(i.e.)  ll1a2 + mm1b2 = nn1

This is the required condition for the lines lx + my + n = 0 and l1x + m1y + n1 = 0 to be conjugate lines.

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Ellipse

7-19

ILLUSTRATIVE EXAMPLES BASED ON TANGENTS, NORMALS, POLE-POLAR AND CHORD Example 7.4 Find the equation of the tangent to the ellipse x2 + 2y2 = 6 at (2, −1).

Solution The equation of the ellipse is x2 + 2y2 = 6. (i.e.) 

x2 y2 + =1 6 3

The equation of the tangent at (x1, y1) is

xx1 yy1 + = 1. 6 3

Therefore, the equation of the tangent at (2, −1) is

2x y − = 1. 6 3

(i.e.)  2x − 2y = 6 ⇒ x − y = 3

Example 7.5 Find the equation of the normal to the ellipse 3x2 + 2y2 = 5 at (−1, 1).

Solution The equation of the ellipse is 3x2 + 2y2 = 5. (i.e.) 

x2 y2 + =1 5 5 3 2

The equation of the normal at (x1, y1) is

ax 2 by 2 − = a2 − b2 . x1 y1



5 x 5 y 5 5 − ⋅ = − . ⋅ 3 ( −1) 2 1 3 2



−5 x 5 y 5 5 −5 x 5 y 10 − 15 − = − ⇒ − = 3 2 3 2 3 2 6 −5 x 5 y −5 ⇒ − = ⇒ 2x + 3y = 1 3 2 6

Therefore, the equation of the normal to the ellipse 3x2 + 2y2 = 5 is 2 x + 3 y = 1.

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7-20

Analytical Geometry

Example 7.6 If B and B′ are the ends of the minor axis of an ellipse then prove that SB = S′B′ = a where S and S′ are the foci and a′ is the semi-major axis. Show also that SBS′B′ is a rhombus whose area is 2abe.

Solution y B(o, b)

S′

C

S

x

B ′(o, −b)

S is (ae, 0); S′ is (−ae, 0) B is (0, b); B′ is (0, −b) SB = ( ae − 0)2 + (0 − b)2 = a 2 e 2 + b 2



= a2 − b2 + b2 = a S ′ B′ = ( − ae − 0)2 + (0 + b)2 = a 2 e 2 + b 2



= a2 − b2 + b2 = a ∴ SB = S′B′ = a. In the figure, SBS′B′ the diagonals SS′ and BB′are at right angles. Therefore, SBS′B′ is a rhombus. Area of the rhombus = 4( ∆CSB)

1 = 4 × × CS ⋅ CB 2 = 2ae ⋅ b = 2abe

Example 7.7

x2 y2 + = 1 meets the major axis at T and PN a2 b2 is the ordinate of P, then prove that CN ∙ CT = a2 where C is the centre of the ellipse. If the tangent at P of the ellipse

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Ellipse

7-21

Solution Let P be the point (x1, y1) xx yy The equation of tangent at (x1, y1) is 21 + 21 = 1. a b When the tangent meets the x-axis, y = 0 ∴

xx 2 a2 = 1 or x = x1 a2 y

P (x1, y1) T

∴ CT =

x

N

C

a2 x1

However, CN = x1 ∴CN ⋅ CT = a 2

Example 7.8 x2 y2 + = 1 meets the tangents at a2 b2 A and A′ (extremities of major axis) in L and M, respectively. Prove that AL ⋅ A′ M = b 2. The tangent at any point P on the ellipse

Solution M

y

P

A′

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 21

C

(a, 0)

L A

x

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7-22

Analytical Geometry

Let the equation of the tangent at P be y = mx + a 2 m2 + b 2 . The equation of the tangent at the point A is x = a. Solving these two equations, we get y = ma + a 2 m2 + b 2 . ∴ AL = a 2 m2 + b 2 + ma



Similarly, A′M = a 2 m2 + b 2 − ma

∴ AL ⋅ A′ M =

(

a 2 m2 + b 2 − ma

)(

a 2 m2 + b 2 + ma

)

= a2 m2 + b2 − m2 a2 = b2 Therefore, AL ⋅ A′ M = b 2.

Example 7.9 If SY and S′Y′ be perpendiculars from the foci upon the tangents at any point x2 y2 of the ellipse 2 + 2 = 1, then prove that Y, Y′ lie on the circle x2 + y2 = a2 a b and that SY ⋅ S ′Y ′ = b 2.

Solution Y′

Y

A′

P A

S′

S

The equation of the tangent at any point P is y = mx + a 2 m2 + b 2



(7.27)

The slope of the tangent is m. Therefore, the slope of the perpendicular line SY is −1. S is the point (ae, 0). m −1 ∴ The equation of SY is y = ( x − ae). m

my + x = ae

(7.28)

Let Y , the foot of the perpendicular, be (x1, y1) Then from equation (7.27), we get

(y1 − mx1)2 = a2 m2 + b2

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 22

(7.29)

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Ellipse

From equation (7.28), we get (my + x1)2 = a2e2 = a2 − b2

7-23

(7.30)

Adding equations (7.29) and (7.30), we get x12 (1 + m2 ) + y12 (1 + m2 ) = a 2 (1 + m2 ). Cancelling, 1 + m2 , x12 + y12 = a 2 The locus of (x1, y1) is x2 + y2 = a2. Similarly, we can prove that the locus of Y′ is also this circle. Hence, Y and Y′ lie on this circle.

SY =

mae + a 2 m2 + b 2



− mae + a 2 m2 + b 2

1 + m2 1 + m2 2 2 2 2 2 2 2 (a m + b ) − m a e a m + b − m2 ( a2 − b2 ) SY ⋅ S ′Y ′ = = 1 + m2 1 + m2 2



, S ′Y ′ =

2

Since (a2 e2 = a2 − b2)

b 2 (1 + m2 ) = b2 2 1+ m Note 7.12.1: This circle is called the auxiliary circle (x2 + y2 = a2). This is the circle described on the major axis as diameter.



=

Example 7.10 x2 y2 If normal at a point P on the ellipse 2 + 2 = 1 meets the major axis at G a b then prove that: (i) CG = e2CN, where C is the centre of the ellipse and N is the foot of the perpendicular from P to the major axis. (ii) SG = eSP where S is the focus of the ellipse.

Solution (i) Let P be the point (x1, y1). a2 x b2 y − = a 2 e 2. The equation of the normal at (x1, y1) is x1 y1 y P

M X

C

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S G

N

x

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7-24

When this meets the x-axis, y = 0. a 2 e 2 x1 ∴x = a2 ∴ CG = e 2 x1 (i.e.) CG = e 2CN



(ii)

Analytical Geometry

SP =e PM a  ∴ SP = ePM = e. NX = e(CX − CN ) = e  − x1  = a − ex1 e  ∴ eSP = ae − e 2 x1 = CS − CG = SG ∴ SG = eSP

Example 7.11 In an ellipse, prove that the tangent and normal at any point P are the external and internal bisectors of the angle SPS′ where Sand S′ are the foci.

Solution P

M′

X′

S′ L

C

M

N

S

X

Let P(x1, y1) be any point on the ellipse. SP =e PM a ∴ SP = ePM = e ⋅ NX = e(CX − CN ) = e  − x1  = a − ex1 e  S′P =e PM ′ ∴ S ′ P = ePM ′ = a + ex1

(7.31)

(7.32)

Let the normal at P meet the x-axis at L. The equation of the normal at P is a2 x b2 y − = a 2 e 2. x1 y1 When y = 0, x = e2x1 ∴L is (e2x1, 0).

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Ellipse

S′L = S′C + CL = ae + e2x1 SL = CS − CL = ae + e2x1



7-25

(7.33) (7.34)

From equations (7.33) and (7.34), we get S ′ P a + ex1 =  SP a − ex1



(7.35)

From equations (7.33) and (7.34), we get S ′ L ae + e 2 x1 a + ex1 = = LS ae − e 2 x1 a − ex1



=



S′P from equation SP

(7.35)

Therefore, the normal PL is the internal bisector of S ′PS. Since the tangent at P is perpendicular to the normal at P, the tangent P is the external bisector.

Example 7.12 x2 y2 + = 1 passing a2 b2 through an end of the minor axis at the centre of the ellipse.

Find the angle subtended by a focal chord of the ellipse

Solution y B b

A′

θ

S

A x

C Q B′

The equation of the ellipse is

x2 y2 + =1 a2 b2

(7.36)

The equation of the focal chord is

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x y + = 1 ae b

(7.37)

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7-26

Analytical Geometry

The combined equation of the lines CB and CQ is got by homogenization of the equation of the ellipse with the help of straight line (7.37).

(i.e.)  x2 a2

(i.e.)

x2 y2  x y  + = +  a 2 b 2  ae b 

2

1  2 xy  =0 1 − 2  − abe e

x 2 (e 2 − 1) 2 xy − =0 ae b

(i.e.)  b(1 − e2) x2 + 2aexy = 0 The angle between the lines CB and CQ is given by tan q = ±

2 h2 − ab ±2 a 2 e 2 − 0 2ae = =± . 2 a+b b(1 − e ) b(1 − e 2 )

 −2ae  Since the angle BCQ is obtuse, q = tan −1  .  b(1 − e 2 ) 

Example 7.13 A bar of given length moves with its extremities on two fixed straight lines at right angles. Prove that any point of the rod describes an ellipse.

Solution Let OA and OB be the two perpendicular lines and AB be the rod of fixed length. Let P(x1, y1) be any point of the rod. Let the rod be inclined at an angle q with OX. y B R O

q

b

P(x, y) a A q Q

x

(i.e.)  OAB = q Take PA = a and PB = b.

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Ellipse

7-27

Then x = OQ = RP = b cos q, y = QP = b sin q, x y ∴ = cos q and = sin q a b x2 y2 + = cos 2q + sin2q = 1 a2 b2 Therefore, the locus of P is an ellipse.

Hence,

Example 7.14 The equation 25(x2 - 6x + 9) + 16y2 = 400 represents an ellipse. Find the centre and foci of the ellipse. How should the axis be transformed so that the x2 y2 ellipse is represented by the equation + = 1? 25 16

Solution 25( x 2 − 6 x + 9) + 16 y 2 = 400 25( x − 3)2 + 16 y 2 = 400 Dividing by 400, Then

( x − 3)2 y 2 + = 1; Take x - 3 = X, y = Y. 16 25

X2 Y2 + = 1. 16 25

The major axis of this ellipse is the Y-axis. ∴ 16 = 25(1 − e2) ⇒ 1 − e 2 =

16 16 9 ⇒ e2 = 1 − = 25 25 25

3 ∴e = . 5 3  Centre is (3, 0). Foci are (3, ± ae) (i.e.)  3, ± 5 ×  (i.e.) (3, ±3). Now shift  5 origin to the point (3, 0) and then rotate the axes through right angles. Then x2 y2 the equation of the ellipse becomes + = 1. 25 16

Example 7.15 Show that if s, s′ are the lengths of the perpendicular on a tangent from the foci, a, a′ those from the verlices and e that from the centre then s, s′ − e2 = e2(aa′ − c2) where e is the eccentricity.

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7-28

Analytical Geometry

Solution Let the equation of the ellipse be x2 y2 + = 1 a2 b2



(7.38)

Foci are S(ae, 0) and S′(−ae, 0). Vertices are A(a, 0) and A′(−a, 0), centre is (0, 0). Any tangent to the ellipse (7.38) is y = mx + a 2 m2 + b 2 . The perpendicular distance from S(ae, 0) is = Similarly, s ′ =

− ae + a 2 m2 + b 2 1 + m2 − ae + a2 m2 + b2

,

a=

,

c=

ae + a 2 m2 + b 2 1 + m2

a + a2 m2 + b2 1 + m2 a2 m2 + b2



a′ =



SS ′ − c 2 =

a2 m2 + b2 − a2 e 2 a2 m2 + b2 a2 e2  − = 2 2 1+ m 1+ m 1 + m2



aa ′ − c 2 =

a2 m2 + b2 − a2 a2 m2 + b2 a2 − = 1 + m2 1 + m2 1 + m2

1 + m2

∴ e 2 ( aa ′ − c 2 ) =

.

1 + m2

a2 e2  1 + m2

(7.39)

(7.40)

From equations (7.39) and (7.40), we get ss′ - c2 = e2(aa′ − c2).

Example 7.16 x2 y2 + = 1. a2 b2 Prove that each common tangent is inclined to the axis at an angle

A circle of radius r is concentric with the ellipse

tan −1

r 2 − b2 and towards its length. a2 − r 2

Solution The equation of the ellipse is

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 28

x2 y2 + =1 a2 b2

(7.41)

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7-29

Ellipse

The equation of the circle concentric with the ellipse is x2 + y2 = r2

(7.42)

y = mx + m2 a 2 + b 2 

(7.43)

y = mx + r 1 + m2 

(7.44)

Any tangent to the ellipse is Any tangent to the circle is

If the tangent is a common tangent then

r 2 (1 + m2 ) = a 2 m2 + b 2 ⇒ m2 ( a 2 − r 2 ) = r 2 − b 2 , m2 =

(i.e.)  tan 2 q =

r 2 − b2 a2 − r 2

r 2 − b2 . a2 − r 2

r 2 − b2 or tan q = a2 − r 2

Therefore, the inclination to the major axis is q = tan −1

r 2 − b2 . a2 − r 2

Example 7.17 Prove that the sum of the squares of the perpendiculars of any tangent of an x2 y2 ellipse 2 + 2 = 1 from two points on the minor axis, each distance a 2 − b 2 a b from the centre is 2a 2.

Solution The equation of the ellipse is

x2 y2 + = 1. a2 b2

Any tangent to the ellipse is y = mx + a 2 m2 + b 2 . The perpendicular dis-

(

)

tance from 0, a 2 − b 2 to the tangent is p1 = ±

(

a2 m2 + b2 − a2 − b2

)

1 + m2

.

The perpendicular distance from 0, − a 2 − b 2 is given by

p2 = ±

a2 m2 + b2 + a2 − b2

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1 + m2

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7-30

Analytical Geometry

p12 + p22



( =



=



=

a2 m2 + b2 − a2 − b2

) +( 2

a2 m2 + b2 + a2 − b2

1 + m2

)

2

2( a 2 m2 + b 2 + a 2 − b 2 ) 1 + m2 2a 2 (1 + m2 ) (1 + m2 )

= 2a 2



Example 7.18 x2 y2 + =1 a2 b2 to the tangent drawn at a point P on the ellipse. If F1 and F2 are the two foci  b2  of the ellipse then show that ( PF1 − PF2 )2 = 4 a 2 1 − 2  .  d 

Let d be the perpendicular distance from the centre of the ellipse

Solution x2 y2 + = 1. Let P(x1, y1) be any point on it. a2 b2 x2 y2 The equation of the tangent at (x1, y1) is 2 + 2 = 1. a b

The equation of the ellipse is

P(x, y)

F2′

C

F1

The perpendicular distance from C on this tangent is d=





⇒

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 30

1 = d

1 2 1 4

x y2 + 14 a b x12 y12 + a4 b4

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Ellipse

7-31

We know that PF1 = a − ex1 , PF2 = a + ex1

( PF1 − PF2 )2 = [( a − ex1 ) − ( a + ex1 )]2 = 4e 2 x12



  x2 y2    b2  4 a 2 1 − 2  = 4 a 2 1 − b 2  14 + 14   b    d  a 



 x 2 a 2 (1 − e 2 ) x12   x 2 b2 x 2   b2  4 a 2 1 − 2  = 4 a 2  12 − 4 1  = 4 a 2  12 −  a4 a   d  a  a



 x 2 (1 − e 2 ) x12  = 4 a 2  12 −  a2 a 



 e2 x 2  = 4 a 2  21  = 4e 2 x12  a 

(7.45)

(7.46)

From equations (7.45) and (7.46), we get

 b2  ( PF1 − PF2 )2 = 4 a 2 1 − 2  .  d 

Example 7.19 Show that the locus of the middle points of the portion of a tangent to the a2 b2 x2 y2 ­ellipse 2 + 2 = 1 included between the axes is the curve 2 + 2 = 4. x y a b

Solution Any tangent to the ellipse

x2 y2 + = 1 is a2 b2

y = mx + a 2 m2 + b 2

(7.47)

When the tangent meets the x-axis, y = 0. ∴x =



− a2 m2 + b2 m

When it meets the y-axis, x = 0.

∴ y = a2 m2 + b2

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7-32

Analytical Geometry

Therefore, the points of intersection of the tangents with the axes are  − a2 m2 + b2  A , 0 and B 0, a 2 m2 + b 2 . Let (x1, y1) be the midpoint of m   line AB.

(

)

− a2 m2 + b2 , 2 y1 = a 2 m2 + b 2 m a2 b2 a2 m2 b2 ∴ 2+ 2 = 2 2 + 4 x1 4 y1 a m + b 2 a 2 m2 + b 2 2 x1 =

a2 m2 + b2 =1 a2 m2 + b2 a2 b2 Therefore, the locus of P(x1, y1) is 2 + 2 = 4. x y =

Example 7.20 x2 y2 Prove that the tangent to the ellipse 2 + 2 = 1 meets the ellipse a b x2 y2 + 2 = a + b in points, tangents at which are at right angles. 2 a b

Solution Any tangent to the ellipse

x2 y2 + = 1 is a2 b2



y = mx + a 2 m2 + b 2

(7.48)

At Q and R let the tangents meet the ellipse x2 y2 + = a + b a b



Q

(7.49)

L(x1, y1)

R

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Ellipse

7-33

Let L( x1 , y1 ) be the point of intersection of tangents at Q and R. Then QR is the chord of contact form L. Its equation is xx1 yy1 + = a + b a b



(7.50)

Equations (7.48) and (7.49) represent the same line. Identifying equations (7.48) and (7.49), we get

m −1 = =  x1   y1   a   b 

a2 m2 + b2 . a+b

am( a + b)



x1 =



y1 = −

a2 m2 + b2 b( a + b) a2 m2 + b2

Therefore, x12 + y12 = ( a + b)2 . The locus of (x1, y1) is the equation of the director circle of the ­ellipse (7.49). However, director circle is the intersection of perpendicular tangents. Hence, the tangents at Q and R are at right angles.

Example 7.21 A chord PQ of an ellipse subtends a right angle at the centre of the ellipse x2 y2 + + 1. Show that the locus of the intersection of the tangents at Q and a2 b2 x2 y2 1 1 R is the ellipse 4 + 4 = 2 + 2 . a b a b

Solution The equation of ellipse is

x2 y2 + +1  a2 b2

(7.51)

P

R

C Q

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7-34

Analytical Geometry

Let R(x1, y1) be the point of intersection of tangents at P and Q. The equation of the chord of contact of PQ is xx1 yy1 + 2 = 1 a2 b



(7.52)

The combined equation of CP and CQ is got by homogenization of equation (7.51) with the help of equation (7.52). 2



x 2 y 2  xx1 yy1  + = + 2 a 2 b 2  a 2 b 



 x2 1   y 2 1  2 x y xy x 2  14 − 2  + y 2  14 − 2  − 12 12 = 0. a  b  ab a b

Since PLQ = 90°, coefficient of x2 + coefficient of y2 = 0. ∴



x12 1 y12 1 − + − =0 a 4 a2 b4 b2

x12 y12 1 1 + = + a 4 b 4 a2 b2 x2 y2 1 1 The locus of P ( x1 , y1 ) is 4 + 4 = 2 + 2 . a b a b ⇒



Example 7.22 x2 y2 Show that the locus of poles with respect to the ellipse 2 + 2 + 1 of any a b x2 y2 1 tangent to the auxiliary circle is 2 + 2 = 2 . a b b

Solution x2 y2 Let (x1, y1) be the pole with respect to the ellipse 2 + 2 + 1. a b The polar of (x1, y1) is

xx1 yy1 + 2 = 1 a2 b ∴

(7.53)

yy1 − xx1 −b 2 x1 b2 + ⇒ = + = 1 y x y1 b2 a2 a 2 y1

This is a tangent to the auxiliary circle x 2 + y 2 = a 2. The condition for that is c 2 = a 2 (1 + m2 ).

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Ellipse

7-35

2

 b4 x 2   b2  a 4 y12 + b 4 x12 b4 (i.e.)    = a 2 1 + 4 12  ⇒ =  y1  y12 a 2 y12  a y1  (i.e.)  b 4 a 2 = a 4 y12 + b 4 x12 Dividing by a 2 b 4 ,

x12 y12 1 + 4 = 2. 4 b a a

The locus of ( x1 , y1 ) is

x2 y2 1 + = . a 4 b 4 a2

Example 7.23 2 2 2 Show that the locus of poles of tangents to the circle ( x − h) + ( y − k ) = r 2 2 2 2 2 x x y y   hx ky  with respect to the ellipse 2 + 2 = 1 is r 2  2 + 2  =  2 + 2 − 1 .  a b b  a b a

Solution

x2 y2 Let ( x1 , y1 ) be the pole with respect to the ellipse 2 + 2 = 1. Then a b xx yy the polar of (x1, y1) is 21 + 21 = 1. This line is a tangent to the circle a b ( x − h)2 + ( y − k )2 = r 2. The condition for this is that the radius of the circle should be equal to the perpendicular distance from the centre on the tangents.

∴r = ±





 hx1 ky1   a 2 + b 2 − 1 x12 y12 + a 4 b2

 x 2 y 2   hx ky  r 2  14 + 14  =  21 + 21 − 1  b  a b a

s

2

2

 x 2 y 2   hx ky  Therefore, the locus of (x1, y1) is r 2  4 + 4  =  2 + 2 − 1 .  b  a b a

Example 7.24 Find the locus of the poles with respect to the ellipse of the tangents to the parabola y2 = 4px.

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7-36

Analytical Geometry

Solution x2 y2 Let (x1, y1) be the pole with respect to the ellipse 2 + 2 + 1. The polar of a b xx yy (x1, y1) is 21 + 21 = 1. a b (i.e.)  y =

−b 2 x1 b2 x+ . 2 y1 a y1

This is a tangent to the parabola y 2 = 4 px. ∴ The condition is

b2 = y1

p .  b 2 x1  − 2  a y  1

(i.e.) 

a 2 py b2 =− 2 1 y1 b x1

(i.e.)  b 4 x1 + a 2 py12 = 0

Therefore, the locus of (x1, y1) is a2 py2 + b4 x = 0.

Example 7.25 Any tangent to an ellipse is cut by the tangents at the extremities of the ­major axis in the point T and T′. Prove that the circle drawn on TT′ as diameter passes through the foci. y T′

T

A′

C

A

x

Solution Let the equation of the ellipse be x2 y2 (7.54) + = 1 a2 b2 The ends of major axis are A(a, 0) and A′(−a, 0). Any tangent to the ellipse is



y = mx + a 2 + m2 + b 2 

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 36

(7.55)

4/22/2013 8:53:06 PM

Ellipse

7-37

This tangent meets the tangents at A, A′ at T and T ′, respectively. Then the co-

(

) (

)

ordinates of T and T ′ are T a, ma + a 2 m2 + b 2 , T ′ − a, ma + a 2 m2 + b 2 . The equation of the circle on TT ′ as diameter is

(

( x − a)( x + a) + y − ma − a 2 m2 + b 2

)( y + ma −

)

a 2 m 2 + b 2 = 0.

x 2 − a 2 + ( y − a 2 m2 + b 2 )2 − a 2 m2 = 0. x 2 − a 2 + y 2 − 2 y a 2 m2 + b 2 + m2 a 2 − a 2 m2 + b 2 = 0. or

x 2 + y 2 − 2( a 2 m2 + b 2 ) y = a 2 − b 2

or

x2 + y2 − 2( a 2 m2 + b 2 ) y = a2e2.

This circle passes through the foci S(ae, 0) and S′(−ae, 0).

Example 7.26 The ordinate NP of a point P on the ellipse is produced to meet the tangent at one end of the latus rectum through the focus S in Q. Prove that QN = SP.

Solution Let LSL′ be the latus rectum through the focus S. The equation of tangent at L is x( ae) y(b 2 /a) (7.56) + = 1 a2 b2 Q P A′

C

N

L S A L′

Let P be the point (x1, y1). The equation of the ordinate at P is x = x1 (7.57) When the tangent at L meets the ordinate at P in Q, the coordinates of Q are given by solving equations (7.56) and (7.57). x1 y (e) + 1 = 1 a a or y1 = a − ex1

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7-38

Analytical Geometry



We know that SP = a – ex1. Therefore, QN = SP.

∴ QN = a − ex1

Example 7.27 The tangent and normal at a point P on the ellipse meet the minor axis in T and Q. Prove that TQ subtends a right angle at each of the foci.

Solution The equation of ellipse is x2 y2 + = 1 a2 b2



T

(7.58)

P(x1, y1)

S

S′ Q

The equation of the tangent and normal at P(x1, y1) is xx1 yy1 + = 1 b2 a2

(7.59)

a2 x b2 y (7.60) − = a 2 − b 2 x1 y1 When the tangent and normal meet in the minor axis in T and Q, respectively,



  b2  ( a 2 − b 2 ) y1  the coordinates of T and Q are T  0,  and Q  0, −  . b2  y1   The coordinates of S are (ae, 0). Slope of

 −b2   b2  TS =   ( − ae) =  = m1  (say)  y1   aey1 

Slope of

QS = −

( a 2 − b 2 ) y1 b2

( − ae) =

a 2 e 2 y1 aey1 = 2 = m2  (say) bae b

Now, m1 m2 = − 1. Therefore, TQ subtends a right angle at the focus S. The coordinates of S′are (−ae, 0). Hence it is proved that TQ subtends a right angle at S′.

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Ellipse

7-39

Example 7.28 x2 y2 + = 1 and e be its eccentricity a2 b2  PSS ′   PS ′ S  1 − e = × tan  then prove that tan  where P is any point on the  2   2  1 + e ellipse. If S and S′ be the foci of the ellipse

Solution x2 y2 + = 1. a2 b2 The coordinates of S are (ae, 0) and S′ are (−ae, 0). ∴ SS′ = 2ae The equation of ellipse is

y P q

f

In any DABC, we know that tan perimeter of DABC.

x

S

S′

A = 2

( s − b)( s − c) where s is the semi s( s − a )

A

c

B

b

a

C

Let PSS ′ = q and PS ′ S = f Then

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SP + S ′ P + SS ′ 2 2a + ae = = a + ae 2

s=

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7-40

Analytical Geometry

∴ tan

tan

q = 2

( a + ae − SS ′ )( a + ae − SP ) ( a + ae)( a + ae − SP ′ )

tan

f = 2

( a + ae − SS ′ )( a + ae − S ′ P ) ( a + ae)( a + ae − SP )

q f a + ae − 2ae a(1 − e) 1 − e ⋅ tan = = = a + ae a(1 + e) 1 + e 2 2

PSS ′   PS ′ S  1 − e Hence, tan  ⋅ tan  =   2  1 + e  2 

Example 7.29 A variable point P on the ellipse of eccentricity e is joined to its foci S and S′. Prove that the locus of the incentre of the ∆PSS′ is an ellipse whose 2e . eccentricity is 1+ e

Solution Let the equation of the ellipse be

x2 y2 + = 1. a2 b2 P(h, k)

S ′( −ae, 0)

S(ae, 0)

The coordinates of the foci are S(ae, 0) and S′(− ae, 0). Let P(h, k) be any point on the ellipse. Then SP + S′P = 2a. Also SS′ = 2ae. Also SP = a − e h, S′P = a + e k. Let the coordinates of the incentre be (x1, y1). Then x1 =

2ae.h + ( a − eh)( − ae) + ( a + eh)ae 2aeh + 2ae 2 h = 2ae + ( a − eh) + ( a + eh) 2a(1+ e)

x1 = 2

aeh (1 + e) 2a (1 + e)

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= eh

(7.61)

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Ellipse

y1 =



2aek + ( a − eh) + ( a + eh) ek = 2a(1 + e) 1+ e

Since (h, k) lies on the ellipse (i.e.) 

(7.62)

x2 y2 h2 k 2 + 2 = 1⇒ 2 + 2 = 1 2 a b a b

x12 y12 = 1. + 2 2 2 ae  be    1+ e

The locus of P(x1, y1) is tricity e1is given by,

7-41

x2 y2 = 1 which is an ellipse whose eccen+ a 2 e 2  be  2   1+ e 2

 be  2 2 2   = a e (1 − e1 ) 1+ e or

b2 = a 2 (1 − e12 ) (1 + e)2

or (1 − e12 ) =



e12 =

b2 a (1 + e)2 2

2e 1+ e

2e 1+ e Therefore, the locus of the incentre of the ∆PSS′ is an ellipse whose eccen­ e1 =

tricity e1 is

2e . 1+ e

Exercises 1.  2. 

Find the equation of the tangent to the ellipse which makes equal intercepts on the axes. Ans.: x + y = a 2 + b 2 Find the length of latus rectum, eccentricity, equation of the directrix and foci of the ellipse 25x2 + 16y2 = 400. 25 32 3 , ,(0, ±3), y = ± Ans.: 5 5 3

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7-42

3.

 4.

Analytical Geometry

The equation to the ellipse is 2x2 + y2 - 8x - 2y + 1 = 0. Find the length of its semi axes coordinates of the foci, length of latus rectum and equation of the directrix. Ans.: 2, 2 3 ,(2, −1),(2, 3), 2 2 , x − 2 = 0, y − 1 = 0 x2 y2 2 Prove that x + y = a + b 2 touches the ellipse 2 + 2 = 1 and find the a b coordinates of the point of contact.  a2 b2  , Ans.:  2   a + b2 a2 + b2 

 5.

6.

7.

8.

9.

10.

x2 y2 If p be the length of the perpendicular from the focus S of the ellipse 2 + 2 = 1 a b b 2 2a on the tangents at P then show that 2 = − 1. p SP If ST be the perpendicular from the focus S on the tangent at any point P on the x2 y2 ellipse 2 + 2 = 1 then show that T lies on the auxiliary circle of the ellipse. a b x2 y2 The line x cos a + y sin a = p intercepted by the ellipse 2 + 2 = 1 subtends a a b ab . right angle at its centre prove that the value of p is

a2 + b2 If the chord of contact of the tangents drawn from the point (a, b ) to the ellipse x2 x2 2 2 2 + = 1 touches the circle x + y = c prove that the point (a, b ) lies on the a2 b2 x2 x2 1 ellipse 2 + 2 = 2 . b c a x2 x2 P is a point on the ellipse 2 + 2 = 1 and Q, the corresponding point on the a b auxiliary circle. If the tangent at P to the ellipse cuts the minor axis in T, then prove that the line QT touches the auxiliary circle. x2 x2 Tangents to the ellipse 2 + 2 = 1 make angles q1 and q 2 with the major axis. a b Find the equation of the locus of their intersection when tan (q 1 + q 2) is a constant.

11.

Show that the locus of the point of intersection of two perpendicular tangents to an ellipse is a circle.

12.

Prove that a chord of an ellipse is divided harmonically by any point on it and its pole with respect to the ellipse.

13.

If the polar of P with respect to an ellipse passes through the point Q, show that polar of Q passes through P.

14.

Find the condition for the pole of the straight line lx + my = 1 with respect to the x2 y2 x2 y2 ellipse 2 + 2 = 1 may lie on the ellipse 2 + 2 = 4. a b a b Ans.: a2l 2 + b2m2 = 4



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Ellipse

15.

7-43

x2 y2 Chords of the ellipse 2 + 2 = 1 touch the circle x2 + y2 = r2. Find the locus of a b their poles.

18.

x2 y2 x2 y2 always touch the ellipse + = 1 + = 1 . Show a2 b2 a2 b2 b2 x 2 a2 y 2 that the locus of the poles is 4 + 4 = 1 . a b Prove that the perpendicular from the focus of an ellipse whose centre is C on any polar of P will meet CP on the directrix. Show that the focus of an ellipse is the pole of the corresponding directrix.

19.

A tangent to the ellipse

20.

If the midpoint of a chord lies on a fixed line lx + my + n = 0, show that the locus  x2 y2  of pole of the chord is the ellipse n  2 + 2 − 1 + lx + my + n = 0. b a  x2 y2 Find the locus of the poles of chords of the ellipse 2 + 2 = 1 which touch the a b parabola ay2 = −2b2x.

16.

17.

21. 22.

23. 24. 25. 26.

Chords of the ellipse

x2 y2 x2 y2 + 2 = 1 meets the ellipse + = a + b at Q and R. 2 a b a b Show that the locus of the pole of QR with respect to the latter is x2 + y2 = a2 + b2.

x2 y2 + = 1 on the polar of a a2 b2 point with respect to the ellipse is equal to c. Prove that the locus of the point is x2 y2 1 the ellipse, 4 + 4 = 2 . b c a Show that the locus of the poles with respect to an ellipse of a straight line which touches the circle described on the minor axis of the ellipse as diameter. x2 y2 Show that the locus of poles of tangents to the ellipse 2 + 2 = 1 with respect a b to x2 + y2 = ab is an equal ellipse. The perpendicular from the centre of the ellipse

Prove that the tangents at the extremities of latus rectum of an ellipse intersect on the corresponding directrix. x2 y2 Find the coordinates of all points of intersection of the ellipse + = 1 and 4 9 2 2 the circle x + y = 6. Write down the equation of the tangents to the ellipse and circle at the point of intersection and find the angle between them.   Ans.: ± 12 , ± 18 , 1 3 x + 1 2 y = 1  5 5  2 5 3 5 

  27.

2

3 3  1  x + 3 y = 6; tan −1   6  5 5

x2 y2 + = 1 to the circle a2 b2 2 2 2 x + y = a . Prove that their chord of contact touches the ellipse a2 x2 + b2 y2 = r4. Tangents are drawn from any point on the ellipse

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7-44

Analytical Geometry

x2 y2 + = 1 and the a2 b2  a − b circle x2 + y2 = ab at their point of intersection is tan −1  .  ab 

28.

Prove that the angle between the tangents to the ellipse

29.

Prove that the sum of the reciprocals of the squares of any two diameters of an ellipse which are at right angles to one another is a constant.

30.

An ellipse slides between two straight lines at right angles to each other. Show that the locus of its centre is a circle.

31.

Show that the locus of the foot of perpendiculars drawn from the centre of the ellipse

x2 y2 + = 1 on any tangent to it is ( x 2 + y 2 )2 = a 2 x 2 + b 2 y 2 . a2 b2

32.

Two tangents to an ellipse interest at right angles. Prove that the sum of the squares of the chords which the auxiliary circle intercepts on them is constant and equal to the square of the line joining the foci.

33.

Show that the conjugate lines through a focus of an ellipse are at right angles.

34.

An archway is in the form of a semi-ellipse, the major axis of which coincides with the road level. If the breadth of the road is 34 feet and a man who is 6 feet high, just reaches the top when 2 feet from a side of the road, find the greatest height of the arch.

35.

If the pole of the normal to an ellipse at P lies on the normal at Q then show that the pole of the normal at Q lies on the normal at P.

36.

37.

38.

39.

x2 y2 PQ, PR is a pair of perpendicular tangents to the ellipse 2 + 2 = 1. Prove that a b x2 y2 1 QR always touches the ellipse 4 + 4 = 2 . a b a + b2 Show that the points (xr, yr), r = 1, 2, 3 are collinear if their polars with respect x2 y2 to the ellipse 2 + 2 = 1 are concurrent. a b x2 y2 If l1 and l2 be the length of two tangents to the ellipse 2 + 2 = 1 at right angles a b 2  2  1 1 2 2 3 2 2 2 to one another, prove that 4( a + b ) = (l1 + l2 )  a + b + a 2b 2  2 + 2   .  l1 l2    If RP and RQ are tangents from an external point R(x1, y1) to the ellipse x2 y2 x2 y2 RS 2 + 2 = 1 and S be the focus then show that = 12 + 12 . 2 a b SP ⋅ SQ a b

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Ellipse

7-45

7.13  ECCENTRIC ANGLE Let P be a point on the ellipse and P′ be the corresponding point on the ­auxiliary circle. The angle CP′ makes with the positive direction of x-axis is called the eccentric angle of the point P on the ellipse. If this angle is denoted by q, then CN = a cos q and NP′ = a sin q. y P′ P q A′

We know that





C

N

A

x

NP b = NP ′ a or b NP = NP ′ a b × a sin q a = b sin q =

Then the coordinates of any point P are (CN, NP). (i.e.)  (a cos q, b sin q ) ∴ ‘q ’ is called the eccentric angle and it is also called the parameter of the point P.

7.14  EQUATION OF THE CHORD JOINING THE POINTS To find the equation of the chord joining the points whose eccentric ­angles are ‘p ’ and ‘e ’: The two given points are (a cos q, b sin q ) and (a cos f, b sin f). The equay − y1 y1 − y2 tion of the chord joining the two points is . = x − x1 x1 − x2

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7-46

(i.e.)

Analytical Geometry

q +f q −f   sin 2 cos y − b sin q b(sin q − sin f ) b  2 2  = =  q +f q −f  x − a cos q a(cos q − cos f ) a sin  −2 sin   2 2  −b = a

q +f  cos   2  q +f  sin   2 

q +f  q +f  q +f  (i.e.) ay sin  − ab sin q sin  = −bx cos  +  2   2   2  q +f  ab cos q cos   2  q +f  q +f  q +f  q +f  (i.e.) bx cos  + ay sin  = ab  cos q cos + sin q sin    2   2  2  2  q +f  = ab cos q −   2    = ab cos

q +f 2

Dividing by ab,

x q +f q +f  y q +f  cos  + sin  = cos    2  b  2  a 2

Therefore, the equation of the chord joining the points whose eccentric angles are ‘q ’ and ‘f’ is

x q +f q +f  y q +f  cos  + sin  = cos .    2  b  2  a 2

Note 7.14.1: This chord becomes the tangent at ‘q ’ if f  = q ∴ The equation of the tangent at ‘q ’ is

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x y cos q + sin q = 1. a b

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Ellipse

7-47

7.15  EQUATION OF TANGENT at ‘p ’ ON THE ELLIPSE x 2 y2 + =1 a 2 b2 The equation of the ellipse is

x2 y2 + = 1. a2 b2

Differentiating with respect to x, we get, 2 x 2 y dy + =0 a 2 b 2 dx



dy b2 x =− 2 dx a y



dy −b 2 a cos q −b cos q at (a cos q , b sin q ) = 2 = dx a sin q a b sin q



The equation of the tangent at ‘q ’ is, −b cos q ( x − a cos q ) a sin q ay sin q + bx cos q = ab(sin 2 q + cos 2 q ) = ab y − b sin q =



Dividing by ab, x y cos q + sin q = 1 a b



The slope of the normal at q is a sin q . b cos q Therefore, the equation of the normal at q is: y − b sin q =

a sin q ( x − a cos q ) b cos q

by cos q − b 2 sin q cos q = ax sin q − a 2 sin q cos q ax sin q − by cos q = ( a 2 − b 2 ) sin q cos q

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7-48

Analytical Geometry

Dividing by sin q cos q, we get, ax by − = a2 − b2 cos q sin q



x2 y2 Therefore, equation of normal at ‘q ’ on the ellipse 2 + 2 = 1 is a b ax bk − = a2 − b2 . cos q sin q

7.16  CONORMAL POINTS In general, four normals can be drawn from a given point to an ellipse. If a, b, g , and d be the eccentric angles of these four conormal points then a + b + g + d is an odd multiple of p. Let (h, k) be a given point. Let P(a cos q, b sin q ) be any point on the ellipse x2 y2 + = 1. a2 b2 ax bk − = a2 − b2 . The equation of the normal at q is cos q sin q If the normal passes through (h, k) then ah bk − = a2 − b2 cos q sin q (i.e.)

 1+ t2   1+ t2  q − = a 2 − b 2 where t = tan ah  bk 2   2 1− t   2t 

(i.e.) ah(1 + t 2 )2t − bk (1 − t 4 ) − ( a 2 − b 2 )2t (1 − t 2 ) = 0 (i.e.)   bkt4 + 2[ah + (a2 − b2)] t3 + 2[ah − (a2 − b2)] t - bk = 0

(7.63)

This is a fourth degree equation in t and hence there are four values for t. For each value of t, there is a value of q and hence there are four values of q say a, b, g, and d. Hence, there are four normals from a given point to an ellipse. Hence, tan

g d a b , tan , tan and tan are the roots of the equation (7.63). 2 2 2 2

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Ellipse

a



∑ tan 2 =



∑ tan



7-49

a b −2[ah + ( a 2 − b 2 )] ; ∑ tan tan = 0 2 2 bk

a b g −2[ah − ( a 2 − b 2 )] tan tan = bk 2 2 2 tan

a b g d tan tan tan = −1 2 2 2 2

(7.64)

S1 − S3 a b g d  We know that, tan  + + +  =  2 2 2 2  1 − S2 + S 4 −2( ab + a 2 − b 2 ) 2( ab − a 2 + b 2 ) + bk bk = 1− 0 −1



=∞

a + b +g +d p = an odd multiples of . 2 2 ∴ a + b + g + d = an odd multipleof p .

∴

7.17  CONCYCLIC POINTS A circle and an ellipse will cut four points and that the sum of the eccentric angles of the four points of intersection is an even multiple of p. Let the equation of the ellipse be x2 y2 + = 1 a2 b2

(7.65)

x2 + y2 + 2gx + 2fy + c = 0

(7.66)

Let the equation of the circle be

Any point on the ellipse is (a cos b, a sin q ). When the circle and the ellipse intersect, this point lies on the circle.

∴a 2 cos 2 q + b 2 sin 2 q + 2 ga cos q + 2 f b sin q + c = 0

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 49

(7.67)

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7-50

Analytical Geometry

q 2 2 = 1− t We know that, cos q = q 1+ t2 1 + tan 2 2 q 2 tan 2 = 2t where t = tan q sin q = q 1+ t2 2 1 + tan 2 2 Substituting these values in equation (7.31), we get 1 − tan 2

2



2

1− t2  1− t2   2t  2  2t  + 2 fb + + 2 a  b ga + c = 0.  2 2 2   1 + t 2  1+ t  1+ t  1+ t  2

(i.e.) ( a 2 − 2 ga + c)t 4 + 4bft 3 + ( 4b2 − 2a2 + 2c)t 2 + 4bft + ( a 2 + 2 ga + c) = 0

where

∑t

∑ t1t2t3 =

1

=



(7.68)

−4bf 4 b 2 − 2 a 2 + 2c ; ∑ t1t2 = 2 a − 2 ga + c a − 2 ga + c 2

−4bf a 2 + 2 ga + c ; . t t t t = 1 2 3 4 a 2 − 2 ga + c a 2 − 2 ga + c

Equation (7.32) is a fourth degree equation in t and hence there are four values for t, real or imaginary. For each value of t there corresponds a value of q. Hence in general there are four points of intersection of a circle and an ellipse with eccentric angles q 1, q 2, q 3, and q 4. We know that,

S1 − S3  q + q + q3 + q4  = tan  1 2 with usual notation.    2 1 − S2 + S 4 =



Σt1 − Σt1t2 t3 =0 1 − Σt1t2 + t1t2 t3t 4

q1 + q 2 + q 3 + q 4 = np 2 ⇒ q1 + q 2 + q 3 + q 4 = 2np ∴



= an even multiple of p .

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Ellipse

7-51

7.18  EQUATION OF A CHORD IN TERMS OF ITS MIDDLE POINT To find the equation of a chord in term of its middle point: Let the equation of the ellipse be x2 y2 + = 1 a2 b2



(7.69)

P R

Q

Let R(x1, y1) be the midpoint of a chord PQ of this ellipse. Let the equation of chord PQ be x − x1 y − y1 = =r cos q sin q



(7.70)

Any point on this line is ( x1 + r cos q , y1 + r sin q ). When the chord meets the ellipse this point lies on the ellipse (7.69).

∴

( x1 + r cos q )2 ( y1 + r sin q )2 + =1 a2 b2

2 2  cos 2 q sin 2 q   x1 cos q y1 sin q  x1 y1 + (i.e.) r 2  + + 2 r + + − 1 = 0  (7.71)  a 2 b 2  b 2  a 2 b 2  a2

If R(x1, y1) is the midpoint of the chord PQ then the two values of r are the distances PR and RQ which are equal in magnitude but opposite in sign. The condition for this is the coefficient of r = 0.

∴

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 51

x1 y cos q + 21 sin q = 0  2 a b

(7.72)

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7-52

Analytical Geometry

Substituting cosq =

x − x1 y − y1 and sinq = in equation (7.69), we get r r x1  x − x1  y1  y − y1  + =0 a 2  r  b 2  r 



xx1 yy1 + 2 = a2 b xx1 yy1 + 2 −1 = a2 b

(i.e.) or

x12 y12 + a2 b2 x12 y12 + −1 a2 b2

Hence, the equation of chord in terms of its middle point is T = S1 where T =

xx1 yy1 x2 y2 + 2 − 1 and S = 12 + 12 − 1 . 2 b a a b

7.19  COMBINED EQUATION OF PAIR OF TANGENTS To find the combined equation of pair of tangents from (x1, y1) to the x 2 y2 ellipse 2 + 2 = 1: a b Let the equation of the chord through (x1, y1) be x − x1 y − y1 = =r cos q sin q



(7.70a)

Any point on this line is (x1 + r cos q, y1 + r sin q ) If this point lies on the ellipse



x2 y2 + = 1, a2 b2

( x1 + r cos q )2 ( y1 + r sin q ) + =1 a2 b2 2  cos 2 q sin 2 q  y2  x cos q y sin q  x + 2  + 2r  1 2 + 1 2  + 12 + 12 − 1 = 0 (i.e.) r 2  2  a b  b  a b  a

The two values of r are the distances of the point of intersection of the chord and the ellipse from (x1, y1). The line will become a tangent if the two values of r are equal. The condition for this is the discriminant of the quadratic equation is zero.

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7-53

Ellipse

2

 x 2 y 2   cos 2 q sin 2 q  y x  + 2  ∴ 12 cos q + 21 sin q  =  12 + 12 − 1  a  b b b  a   a2



Using the values of cos q  and sin q from equation (7.37), 2



2 2   ( x − x1 )2 ( y − y1 )2   x1 ( x − x1 ) y1 ( y − y1 )   x1 y1 =  2 + 2 − 1  + +  2 2   a b b b2   a2  a  

(i.e.)  xx1 yy1  x12 y12    x12 y12   x 2 y 2  xx1 yy1  x12 y12   2 + 2 −  2 + 2   =  2 + 2 − 1  2 + 2 − 2  2 + 2  + 2 + 2  a b b  a b  b a b b    a  a  a 2

2  xx yy  x 2 y 2   x 2 y 2  y2    x (i.e.)  21 + 21 − 1 −  12 + 12 − 1  =  12 + 12 − 1  2 + 2 − 1  a b b b b     a a   a 2 y2    xx yy  x −2  21 + 21 − 1 +  12 + 12 − 1  a  a b b  

(i.e.)  (T − S1 )2 = S1 ( S − 2T + S1 )

T 2 − 2TS1 + S12 = SS1 − 2TS1 + S12

(i.e.)

T 2 = SS1 where T =

xx1 yy1 + 2 −1 a2 b x2 y2 + −1 a2 b2 x2 y2 S1 = 12 + 12 − 1 a b S=



This is the combined equation of the pair of tangents from (x1, y1).

Note 7.19.1: The combined equation of the pair of tangents from the point (x1, y1) is 2



 x 2 y 2   x12 y12   xx1 yy1   a 2 + b 2 − 1 =  a 2 + b 2 − 1  a 2 + b 2 − 1 .   

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7-54

Analytical Geometry

If the two tangents are perpendicular then coefficient of x2 + coefficient of y2 = 0.  x12 1  x12 y12  y12 1  x12 y12    a 4 − a 2  a 2 + b 2 − 1 + b 4 − b 2  a 2 + b 2 − 1       2 2   x − y1  (ii.e.)  + 12 − 2 1 2 + 12 = 0 2 2 ab a a1 b b     2 2 + x y 1 1 + 1 ⇒ x 2 + y 2 = a2 + b2   (i.e.) 1 = 1 1   a2 b2 a2 b2 The locus of (x1, y1) is x2 + y2 = a2 + b2. Therefore, the locus of the point of intersection of perpendicular tangents is a circle. This equation is called the directrix of the circle.

7.20  CONJUGATE DIAMETERS Example 7.30 Find the condition that the line lx + my + n = 0 may be a tangent to the ellipse x2 y2 + = 1. a2 b2

Solution x2 y2 + = 1. a2 b2 Let the line be tangent at ‘q ’. The equation of the tangent at q  is

Let lx + my + n = 0 be a tangent to the ellipse



x y cos q + sin q = 1  a b

(7.71a)

However, the equation of tangent is given as lx + my = − n



(7.72a)

Identifying equations (7.71a) and (7.72a), we get



1 l m = = q q cos sin  −n        a   b  ∴ cosq =

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 54

− al , n

sinq =

−bm . n

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7-55

Ellipse

Squaring and adding, we get cos 2 q + sin 2 q =



(i.e.) 

a2 l 2 b2 m2 a2 l 2 b2 m2 + ⇒ 1 = + 2 n2 n2 n2 n

a2l2 + b2m2 = n2.

This is the required condition.

Example 7.31 Find the condition for the line lx + my + n = 0 to be a normal to the ellipse x2 y2 + = 1. a2 b2

Solution The equation of the ellipse is The equation of normal is

x2 y2 + = 1. a2 b2 lx + my = -n



(7.73)

Let this equation be normal at ‘q ’. The equation of the normal at ‘q ’ is ax by − = a2 − b2 cos q sin q The equations (7.73) and (7.74) represent the same line. Therefore, identifying equations (7.73) and (7.74), we get



−n l m = =  a   b  a2 − b2     cos q   sin q 





(7.74)

∴ cos q =

− an −bn , sin q = l( a2 − b2 ) m( a 2 − b 2 )

Squaring and adding equations (7.78) and (7.79), we get

a 2 n2 b 2 n2 + 2 2 =1 2 2 2 l (a − b ) m ( a − b2 )2 2

∴

a2 b2 ( a2 − b2 )2 + = l 2 m2 n2

This is required condition.

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7-56

Analytical Geometry

Example 7.32 Show that the locus of the point of intersection of tangents to an ellipse at the points whose eccentric angles differ by a constant is an ellipse.

Solution Let the eccentric angles of P and Q be a + b and a - b. ∴ (a + b ) – (a - b ) = 2b = 2k; a constant ∴b = k. The equation of tangents at P and Q are x y cos(a + b ) + sin(a + b ) = 1  a b x y cos(a − b ) + sin(a − b ) = 1  a b

(7.75) (7.76)

Let (x1, y1) be their point of intersection. Then

x1 cos(a + b ) + a x1 cos(a − b ) + a

y1 sin(a + b ) = 1 b y1 sin(a − b ) = −1 b

(7.77) (7.78)

Solving equations (7.77) and (7.78), we get x1 a y1 b = −[sin(a − b ) − sin(a − b )] cos(a + b ) − cos(a − b ) 1 = cos(a + b ) sin(a − b ) − sin(a + b ) cos(a − b ) x1 y1 1 −1 ⇒ = = = −2a cos a sin b −2b sin a sin b − sin 2 b 2 sin b cos b a cos a b sin a , y1 = ∴ x1 = cos b cos b x cos b y cos b ∴ cos a = 1 , sin a = 1 a b Squaring and adding, we get x12 cos 2 b y12 sin 2 b + a2 b2 x cos 2 k y sin 2 k + = 1 which is an Since b  = k, the locus of (x1, y1) is a2 b2 ellipse.

1=

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Ellipse

7-57

Example 7.33

x2 y2 Show that the locus of poles of normal chords of the ellipse 2 + 2 = 1 is a a a6 b6 2 2 2 a b + = ( − ) . x2 y2

Solution Let (x1, y1) be the pole of the normal chord of the ellipse x2 y2 + = 1 a2 b2 Then the polar of (x1, y1) with respect to ellipse is

(7.79)

xx1 yy1 + 2 = 1 (7.80) a2 b Let this be normal at ‘q ’ on the ellipse of equation (7.84). Then the equation of the normal at ‘q ’ is ax by (7.81) − = a2 − b2  cos q sin q Equations (7.80) and (7.81) represent the same line.



Therefore, identifying equations (7.80) and (7.81), we get x y a2 = b2 = 1 a −b a2 − b2 cos q sin q



(i.e.)

x1 cos q − y1 sin q 1 = = 2 3 3 b a − b2 a 3 a ∴ cos q = x1 ( a 2 − b 2 ) sin q =



− b3 y1 ( a 2 − b 2 )

Squaring and adding we get,

1=

a6 b6 + x12 ( a 2 − b 2 )3 y12 ( a 2 − b 2 )2

Therefore, the locus of (x1, y1) is

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 57

a6 b6 + = ( a2 − b2 )2 . x2 y2

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7-58

Analytical Geometry

Example 7.34 Find the locus of midpoints of the normal chords of the ellipse

x2 y2 + = 1. a2 b2

Solution Let (x1, y1) be the midpoint of a chord of the ellipse which is normal at q. The equation of the chord in terms of its middle point is xx1 yy1 x12 y12 + 2 = 2 + 2 (7.82) a2 b a b The equation of the normal at ‘q ’ is ax by − = a2 − b2 cos q sin q Equations (7.82) and (7.83) represent the same line. Therefore, identifying equations (7.87) and (7.88) we get,







(7.83)

x1 y1 x12 y12 + 2 2 a2 b = 2 = a 2 b2 a b a −b cos q sin q x1 x12 + x1 cos q y1 sin q a2 b 2 = = a2 − b2 a3 b3  x2  x2 y2  y2  b3  1 + 12  a3  1 + 12  b  b   a2  a2 cos q = , sinq = 2 2 2 y1 ( a − b 2 ) x1 ( a − b )

Squaring and adding, we get 2





 x12 y12  + 2 a6  a2 b  b3 + 2 2 2 2 2 x1 ( a − b ) y1 (i.e)

 x12 y12   a + b 2  2

2

( a 2 − b2 )2

=1

( a2 − b2 )2 a6 b6 + = x12 y12  x 2 y 2  2 1 1  a + b 2  2 2

 a6 b6   x12 y12  2 2 2  x 2 + y 2   a 2 + b 2  = ( a − b ) 1 1

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Ellipse

7-59

2

 a6 b6   x 2 y 2  Therefore, the locus of (x1, y1) is  2 + 2   2 + 2  = ( a 2 − b 2 )2. y a b  x

Example 7.35 If the chord joining two points, whose eccentric angles are a and b on the x2 y2 ellipse 2 + 2 = 1 cuts the major axis at a distance d from the centre, show a b a b d −a . that tan ⋅ tan = b a d+a

Solution The equation of the chord joining the points whose eccentric angles are a and x a +b a −b a + b  y + sin = cos b  is cos  .   2  b a 2 2 This line meets the major axis at the point (d, 0). d (a − b ) a + b  ∴ cos  = cos  2  a 2 a cos   d ∴ = a a cos  

d −a ⇒ d+a





∴tan

a b tan 2 2

−b  2  +b  2  a a −b cos − cos 2 = a −b a cos + cos 2 a b 2 sin sin 2 2 = a b 2 cos cos 2 2 a b = tan tan 2 2 d −a = d+a

+b 2 +b 2

Example 7.36 The tangent at the point a on the ellipse meet auxiliary circle on two points which subtend a right angle at the centre. Show that the eccentricity of the ellipse is (1 + sin 2 a ) −1/ 2.

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7-60

Analytical Geometry

Solution Q a

P

C

Let the equation of the ellipse be x2 y2 + = 1 a2 b2



(7.84)

The equation of the auxiliary circle is x2 + y2 = a2



(7.85)

x y cos a + sin a = 1. This line meets the a b auxiliary circle at P and Q. Then the combined equation of the lines CQ and 2  x cos a y  CR is x 2 + y 2 = a 2  + sin a  .  a  b

The equation of the tangent at a is

(i.e.)

x 2 [1 − cos 2 a ] −

  a2 2 xya sin a cos a + y 2 1 − 2 sin 2 a  = 0 b b  

Since QCR = 90°, coefficient of x2 + coefficient of y2 = 0 a2 sin 2 a = 0 b2 sin 2 a sin 2 a + 1 − =0 1 − e2 (1 − e 2 ) sin 2 a + (1 − e 2 ) − sin 2 a = 0

∴



1 − cos 2 a + 1 −

e 2 (1 + sin 2 a ) = 1 ⇒ e 2 =

1 1 + sin 2 a

⇒ e = (1 + sin 2 a ) −1/ 2

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Ellipse

7-61

Example 7.37 If the normal at the end of a latus rectum of an ellipse passes through one extremity of the minor axis, show that the eccentricity of the curve is given by e 4 + e 2 − 1 = 0.

Solution Let the equation of the ellipse be

x2 y2 + = 1. a2 b2 2 L(ae, ba )

S L′

 b2  The coordinates of the end L of the latus rectum are  ae,  . The equation a  a2 x b2 y of the normal at L is − = a2 − b2 . x1 y1  b2  This normal passes at the point  ae,  . a  ∴

a2 x b2 − 2 = a2 − b2 ae ba

This line passes through the point (0, −b). ∴

b3 b2 a

= a 2 e 2 ⇒ ab = a 2 e 2

2 2 4 4 2 2 2 4 4 Squaring a b = a e ⇒ a ( a (1 − e )) = a e



⇒ 1 − e2 = e4



⇒ e4 + e2 − 1 = 0

Example 7.38 Prove that the tangent and normal at a point on the ellipse bisect the angle between the focal radii of that point.

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7-62

Analytical Geometry

P T C

S′

G S

Solution Let the equation of the ellipse be x2 y2 + = 1 a2 b2



(7.86)

Let PT and PQ be the tangent and normal at any point P on the ellipse. The a2 x b2 y equation of the normal at ( x1 , y1 ) is 1 − 1 = a 2 − b 2 . x y When this meet the major axis, y = 0. ∴

a2 x = a 2 e 2 ⇒ x = e 2 x1 x1 ∴ G is (e 2 x1 , 0)



S ′G SC + CG ae + e 2 x1 a + ex1 S ′ P = = = = GS CS − CG ae − e 2 x1 a − ex1 SP

Since SP ′ = a + ex1 and SP = a − ex1. ∴

S ′G S ′ P = PS GS

Hence, PG bisects internally S ′ PS . Since the tangent PT is perpendicular to SG, PT is the external bisector of S ′ PS . Therefore, the tangent and normal at P are the bisectors of the angles between the focal radii through that point.

Example 7.39 x2 y2 + =1 a2 b2 2 x 2 y 2 a2 + b2  x 2 y 2  which subtends a right angle at the centre is 4 + 4 = 2 2  2 + 2  . a b a b a b 

Show that the locus of the middle point of chord of the ellipse

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7-63

Ellipse

Solution The equation of the ellipse is

x2 y2 + = 1 a2 b2



(7.87)

Let ( x1 , y1 ) be the midpoint of a chord of the ellipse of equation (7.87). P

C

R(x1, y1) Q

Then its equation is

xx1 yy1 x12 y12 + 2 = 2+ 2. a2 b a b

If C is the centre of the ellipse, the combined equation of the lines CP and CQ is   x y + 2 = 2 a b   2

2

2

xx1 yy1  + 2  a2 b  . x12 y12  + a2 b2  2

 x2 y2   x2 y2   xx yy  ⇒  2 + 2   12 + 12  =  21 + 21  a b a b  b  a

2

Since PCQ = 90°, coefficient of x2 + coefficient of y2 = 0.

1 a2

2

2

 x12 y12  x12 1  x12 y12  y12  a 2 + b 2  − a 4 + b 2  a 2 + b 2  − b 4 = 0.

x2 y2  1 1   x2 y2  The locus of ( x1 , y1 ) is 4 + 4 =  2 + 2   2 + 2  a a b b a b 

2

x 2 y 2 a2 + b2  x 2 y 2  + = 2 2  2 + 2 a4 b4 a b a b 

2



(i.e)

Example 7.40 Prove that the portion of the tangent to the ellipse intercepted between the curve and the directrix subtends a right angle at the corresponding focus.

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7-64

Analytical Geometry

P T

S

x = ae

Solution

x2 y2 Let P be the point ( a cos q , b sin q ) on the ellipse 2 + 2 = 1. The equation a b of the tangent at q is x y (7.88) cos q + sin q = 1 a b The equation of the corresponding directrix is a x=  (7.89) e Solving equations (7.88) and (7.89), we get T, the point of intersection.



a cos q y y 1 e + sin q = 1 ⇒ sin q = 1 − cos q a b b e y e − cos q b(e − cos q ) ∴ sin q = ⇒y= b e e sin q  a b(e − cos q )  ∴ T is  ,  e e sin q 

The slope of SP is

b sin q = m1. a(cos q − e)

 b(e − cos q )    e sin q  = m2. The slope of ST is a − ae e ∴ m2 =

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b(e − cos q ) a sin q (1 − e 2 )

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Ellipse



m1m2 =

7-65

b sin q b(e − cos q ) a 2 (1 − e 2 ) sin q (e − cos q ) × = a(cos q − e) a sin q (1 − e 2 ) a(cos q − e)a sin q (1 − e 2 )

= −1 p ∴ PST = 2

Example 7.41 x2 y2 + = 1 is drawn. It a2 b2 meets the major and minor axis in P and Q respectively. If C is the centre of A normal inclined at 45° to the x-axis of the ellipse

the ellipse, show that the area of ∆CPQ is

( a2 − b2 )2 sq. units. 2( a 2 + b 2 )

Solution The equation of the normal at ‘q ’ is ax by − = a2 − b2 cos q sin q When this meets x-axis, y = 0. x=



(a

2

)

− b 2 cosq a

 ( a − b ) cos q  , 0 . Therefore, P is  a   When it meets y-axis, x = 0. 2

2

y=



−( a 2 − b 2 ) sin q b

 −( a 2 − b 2 ) sin q  Therefore, Q is 0, . b   C is (0, 0). Slope of the normal =

Area of the ∆CPY =

a sin q a = tan 45° ⇒ tan q = 1 b cos q b b ∴ tan q = . a

1 ( x1 y2 − x2 y1 ) 2

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7-66

Analytical Geometry





=

1 ( a 2 − b 2 )2 sin q cos q ab 2

=

b a 1 ( a2 − b2 )2 ⋅ ⋅ 2 2 2 ab 2 a + b2 a +b

=

( a2 − b2 )2 sq.units. 2( a2 + b2 )

Example 7.42 If a - b is a constant, prove that the chord joining the points, ‘a’ and ‘b ’ touches a fixed ellipse.

Solution The equation of the chord joining the points a and b is x a −b a + b  y a + b  cos  + sin  = cos .  2  b  2  a 2 (i.e.) 

x a + b  y a + b  cos   + sin   = cos  k where a - b = 2k.  a b 2 2 

a +b =f, Take 2 y sin q = cos k . b

then

the

(i.e.) 

above

equation

becomes

x cosq + a

x cos q y sin q + = 1. a cos k b cos k

This line is a tangent to the ellipse

x2 y2 + = 1. ( a cos k )2 (b cos k )2

Example 7.43

x2 y2 + =1 a2 b2 q f −b 2 subtends a right angle at the point (a, 0) then show that tan ⋅ tan = 2 . 2 2 a

If the chord joining the variable points at q and f on the ellipse

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Ellipse

7-67

Solution P

A(a, 0)

Q

P is the point ( a cos q , b sin q ). Q is the point ( a cos f , b sin f ). Slope of AP is

b sin q . a(cos q −1)

Slope of AQ is

b sin f . a(cos f −1)

Since AP is perpendicular to AQ, b sin q b sin f ⋅ = −1 a(cos q − 1) a(cos f − 1)



(i.e.) 

b2 ∴ 2⋅ a

b2 sin q sin f = −1 a 2 (1 − cos q )(1 − cos f ) q q f f sin sin cos 2 2 2 2 = −1 2q 2 f 2 sin ⋅ 2 sin 2 2 f q cos cos b2 2 2 = −1 ⇒ 2 q f a sin sin 2 2

4 cos





∴ tan

q f −a 2 tan = 2 2 2 b

Example 7.44 x2 y2 If the normal to the ellipse + = 1 at the point a cuts the curve join in −2 14 5 2a  show that cos a = . 3

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7-68

Analytical Geometry

Solution x2 y2 + = 1. 14 5 a 2 = 14, b 2 = 5 The equation of the normal at ‘a ’ is

The equation of the ellipse is

ax by − = a2 − b2 cos a sin a This passes through the point 2a. \

a b ⋅ a cos 2a − ⋅ b sin 2a = a 2 − b 2 cos a sin a 14 cos 2a 5 sin 2a − = a2 − b2 cos a sin a



14(2 cos 2 a − 1)(cos2 a − 1) 5(2 sin a cos a ) − = 14 − 5 = 9 cos a sin a



(i.e.)  28 cos 2 a − 14 − 10 cos 2 a = 9 cos a .



18 cos 2 a − 9 cos a − 14 = 0. (6 cos a − 7)(3 cos a + 2) = 0 or cosa =



As cos a < 1, cos a =

−2 3

−2 7 or 3 6

Example 7.45 x2 y2 + = 1 meets the major and a2 b2 minor axes in G and g and if CF be the perpendicular upon this normal, where If the normal at any point P to the ellipse

C is the centre of the ellipse, then prove that PF ⋅ Pg = a 2 and PF ⋅ PG = b 2 .

Solution y L P A

C g

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 68

F

G

A′ x

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Ellipse

7-69

Let P ( a cos q , b sin q ) be any point on the ellipse. Let the normal at P meet the major axis in G and minor axis in g. Let CF be the perpendicular from C to the normal at P. The equations of the tangent and normal at P are x y cos q + sin q = 1 a b



(7.90)

ax by − = a2 − b2 cos q sin q



(7.91)  ( a 2 − b 2 ) cos q  , 0  a  

 hen the coordinates of G and g are T  −( a 2 − b 2 ) sin q   0, n  

and

PF = CL where CL is the perpendicular on the tangent.

=



=







cos q sin 2 q + 2 a2 b ab



(7.92)

a sin q + b 2 cos 2 q 2

(

2

)

2

 a 2 − b 2 cos q  b 4 cos 2 q 2 2 + b 2 sin 2 q . PG =  a cos q −  + b sin q =   a a2 2

=

∴ PF ⋅ Pg =

1 2

b 2 (b 2 cos 2 q + a 2 sin 2 q ) a2 ab

b 2 2 a sin q + b 2 cos 2 q = b 2 a sin q + b cos q a 2

2

2

2

⋅

  a2 − b2 Pg 2 = a 2 cos 2 q +  b sin q + sin q  b   = a 2 cos 2 q + PF ⋅ Pg =

a 4 sin 2 q a 2 (b 2 cos 2 q + a 2 sin 2 q ) = b2 b2

ab a sin q + b cos q 2

2

2

2

2

⋅

a 2 2 a sin q + b 2 cos 2 q b

= a2

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7-70

Analytical Geometry

Example 7.46 Show that the condition for the normals at the points ( xi , yi ) , i = 1, 2, 3 on the x1 x2 y2 ellipse 2 + 2 = 1 to be concurrent is x2 a b x3

y1 y2 y3

x1 y1 x2 y2 = 0. x3 y3

Solution Let (h, k) be the point of concurrence of the normal. The equation of the a2 x b2 y normal at ( xi , yi ) is − = a 2 − b 2 (7.93) x1 y1 Since this normal passes through (h, k), a2 h b2 k − = a2 − b2 x1 y1

(i.e.)  b 2 kx1 − a 2 hy1 + ( a 2 − b 2 ) x1 y1 = 0.

(7.94)

Similarly,

b 2 kx2 − a 2 hy2 + ( a 2 − b 2 ) x2 y2 = 0 

(7.95)



b 2 kx3 − a 2 hy3 + ( a 2 − b 2 ) x3 y3 = 0. 

(7.96)

Eliminating h and k from equations (7.94), (7.95) and (7.96), we get b 2 x1 b 2 x2 b3 x3



− a 2 y1 − a 2 y2 − a 2 y3

( a 2 − b 2 ) x1 y1 ( a 2 − b 2 ) x2 y2 = 0 ( a 2 − b 2 ) x3 y3



x1 (i.e.)  b ( − a )( a − b ) x2 x3

y1 y2 y3

x1 y1 x2 y2 = 0. x3 y3



x1 x2 x3

y1 y2 y3

x1 y1 x2 y2 = 0. x3 y3

2

2

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2

2

(i.e.) 

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7-71

Ellipse

Example 7.47 Show that the area of the triangle inscribed in an ellipse is a −b b −g g −a where a, b  and g  are the eccentric ­angles 2ab sin sin sin 2 2 2 of the vertices and hence find the condition that the area of the triangle inscribed in an ellipse is maximum.

Solution Let D ABC be inscribed in the ellipse x2 y2 + =1 a2 b2

B

(7.97)

A

C

Let A, B and C be the points (a cos a, b sin a), (a cos b, b sin b ) and (a cos g, b sin g ), respectively. Then the area of the D ABC is given by, a cos a 1 ∆ ABC = a cos b 2 a cos q



cos a ab = cos b 2 cos g





=



= =

b sin a 1 b sin b 1 b sin q 1 sin a 1 sin b 1 sin g 1

ab [1(cos a sin b − sin a cos b ) − 1(cos a sin g − sin a cos g ) + 2 1(cos b sin g − sin b cos g )] ab [sin( b − g ) + sin(g − a ) + sin(a − b )] 2

a −b ab b −g g −a since b − g + g − a + a − b = p 4 sin sin sin 2 2 2 2

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7-72

Analytical Geometry

If A′, B′ and C′ are the corresponding points on the auxiliary circle then ∆ A′ B′C ′ = 2a 2 sin



b −g g −a a −b sin sin 2 2 2

∆ ABC b = ∆ A′ B′C ′ a b (i.e.)  D ABC = ∆A′ B′C ′ a ∴



Area of D ABC is the greatest when the area of ∆ A′ B′C ′ is the greatest. However, the area of A′ B′C ′ is the greatest when the triangle is equilateral. In this 2p 4p case the eccentric angles of the points P, Q and R are a , a + ,a + . 3 3 2p (i.e.)  The eccentric angles of the points P, Q and R differ by . 3

Example 7.48 If three of the sides of a quadrilateral inscribed in an ellipse are in a fixed direction, show that the fourth side of the quadrilateral is also in a fixed direction.

Solution Let a, b, g  and d  be the eccentric angles of the vertices of the quadrilateral ABCD inscribed in the ellipse x2 y2 (7.98) + =1 a2 b2 Then the equation of the chord PQ is The slope of the chord PQ is

x a +b y a +b a −b . cos + sin = cos a 2 b 2 2

−b a +b . cot a 2

Since the direction of PQ is fixed, Similarly,

\

−b a +b = a constant. cot a 2

a +b = k, a constant. 2

g +d b +g = k3 . = k 2 and 2 2

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Ellipse

Adding

7-73

a + 2 b + 2g + d = k1 + k 2 + k3 2

(i.e.) 

a +d + b + g = k1 + k 2 + k3 2 (i.e.) 



a +d = k1 + k 2 + k3 − 2k 2 2 = k1 + k2 - k3 = k (a constant)

Therefore, the direction of PS is also fixed.

Example 7.49 Prove that the area of the triangle formed by the tangents at the points a, b and g is a −b b −g g −a ⋅ tan . ab tan tan 2 2 2

Solution The equation of tangents at a  and b  are x y cos a + sin a = 1 a b x y cos b + sin b = 1 a b (i.e.)

bx cos a  + ay sin a − ab = 0 bx cos b  + ay sin b − ab = 0

(7.99) (7.100)

Solving equations (7.104) and (7.105) we get, x y 1 = 2 = a b(sin b − sin a ) ab (cos b − cos a ) ab(cos a sin b − sin a cos b ) 2

(i.e.) 

x y 1 = = a(sin a − sin b ) b(cos a − cos b ) sin(a − b )

a(sin a − sin b ) \ x = = sin(a − b )

M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 73

a −b a +b sin 2 2 a −b a −b 2 ⋅ sin ⋅ cos 2 2

a ⋅ 2 cos

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7-74

Analytical Geometry

a + b  a cos   2  x= a − b  cos   2  a + b  b ⋅ sin   2  y= a − b  cos   2  Therefore, the point of intersection of tangents at P is  a + b  a +b   a cos  2  b sin  2   , a −b  a − b    cos  2  cos 2    Hence, the area of the triangle formed by the tangents at a, b and g is a +b 2 b +g 1 1 a cos ∆= b −g g −a a −b 2 2 cos cos cos 2 2 2 g +a a cos 2 a cos

∆=





1 2

a +b 2 b +g b sin 2 g +a b sin 2

b sin

a −b 2 b −g cos 2 g −a cos 2

cos

ab a +b a −b  cos sin a −b b −g g − a  ∑ 2 2  cos cos cos 2 2 2   ab  sin(a − b ) + sin( b − g ) + sin(g − a )  =   a −b b −g g −a 4   cos cos cos  2 2 2  g −a a −b b −g  4 sin sin sin ab  2 2 2 =  a −b b −g g −a 4  cos cos cos  2 2 2 g −a a −b b −g = ab tan tan tan 2 2 2

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    

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Ellipse

7-75

Exercises 1.

2.

3.

If a  and b  be the eccentric angles at the extremities of a chord of an ellipse of a +b a − b  . eccentricity e, prove that cos  = e ⋅ cos  2  2 x2 y2 Let P and Q be two points on the major axis of an ellipse 2 + 2 = 1 a b equidistant from the centre. Chords are drawn through P and Q meeting the ellipse at points whose eccentric angles are a, b, gand d. Then prove that a b g d tan tan tan tan = 1. 2 2 2 2 x2 y2 Prove that the chord joining the points on the ellipse 2 + 2 = 1 whose a b 2p touches another ellipse whose semi-axes are half eccentric angles differ by 3 those of the first. x2 y2 + = 1 such that PQ is a a2 b2 diameter. Prove that P′Q′ passes through a fixed point on the major axis of the 2ae  ellipse. Find also its equation. Ans.:  2 ,0  e + 1 

4.

PSP′ and QSQ′ are two focal chords of ellipse

5.

P and P′ are the corresponding points on an ellipse and its auxiliary circle. Prove that the tangents at P and P′ intersect on the major axis.

6.

The tangent at one end P of a diameter PP′ of an ellipse and any chord P′Q through the other end meet at R. Prove that the tangent at Q bisects PR.

7.

Prove that the three ellipses a12 a common tangent if a22 a32

x2 y2 x2 y2 x2 y2 + 2 = 1, 2 + 2 = 1 and 2 + 2 = 1 will have 2 a1 b1 a3 b3 a2 b2

b12 1 b22 1 = 0. b32 1

8.

Any tangent to the ellipse is cut by the tangents at the ends of the major axis in T and T ′. Prove that the circle on TT ′ as diameter will pass through the foci.

9.

Find the coordinates of the points on the ellipse

x2 y2 + = 1, the tangents at a2 b2 which will make equal angles with the axis. Also prove that the length of the a2 + b2 . perpendicular from the centre on either of these is 2



M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 75

 a2 b2  Ans.:  ± ,±  2 2 2  a +b a + b2 

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7-76

10.

11.

12.

13.

Analytical Geometry

Find the condition for the line x cos a + y sin a = p is a tangent to the ellipse x2 y2 Ans.: a  cos 2a + b sin2a = p2 + = 1.  a2 b2 x2 y2 + = 1, intercepts lengths a and b on the a2 b2 a1 b1 coordinate axes then show that 2 + 2 = 1 . a b x2 y2 If x cos a + y sin a - p = 0 be a tangent to the ellipse 2 + 2 = 1, prove that p2 = a b a2 cos 2a + b2 sin a. If P be the point of contact of the tangent x cos a + y sin a = p and N, the foot of the perpendicular on it, from the centre of the ellipse, prove a2 − b2 sin 2a . that pNP = 2 The tangent at one end of P of a diameter OP′ of an ellipse and any chord P′Q through the other end meet in R. Prove that the tangent at Q bisects OR. If the tangent to the ellipse

14.

P and P′ are corresponding points on an ellipse and the auxiliary circle. Prove that the tangents at P and P′ intersect on the major axis.

15.

If the normal at a point P on the ellipse of semi-axes a, b and centre C cuts the major and minor axes at G and g, show that a2Cg2 + b2CG2 = (a2 - b2)2. Also prove that PG = e·GN, where PN is the ordinate of P.

16.

The tangents and normal at a point P on the ellipse

17.

Prove that, the line joining the extremities of any two perpendicular diameters of an ellipse always touches a concentric circle.

18.

Show that the locus of the foot of the perpendicular drawn from the centre of x2 y2 the ellipse 2 + 2 = 1 on any tangent to it is (x2 + y2)2 = (a2x2 + b2y2)2. a b x2 y2 If P is any point on the ellipse 2 + 2 = 1 whose ordinate is y′, prove that the a b  b2  angle between the tangent at P and the fixed distance of P is tan −1  .  a ⋅ ey′  Show that the feet of the normals that can be drawn from the point (h, k) to the x2 y2 ellipse 2 + 2 = 1 lie on the curve b2(k - y) + a2y (x - h) = 0. a b x2 y2 If the normals at the four points (xi, yi), i = 1, 2, 3, 4 on the ellipse 2 + 2 = 1 a b  1  1 are concurrent show that: (i) ( Σx1 )  Σ  = 4 and (ii) ( Σy1 )  Σ  = 4 .  x1   y1 

19.

20.

21.

x2 y2 + = 1 meet the major a2 b2 axis in T and T ′ so that TT ′ = a. Prove that the eccentric angle of P is given by e2cos 2q + cos q - 1 = 0.

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Ellipse

22.

23.

7-77

If the normals at the four points q i, i = 1, 2, 3, 4 are concurrent, prove that (S  cos  qi)(S secqi) = 4. Show that the mean position of these four points is  1 a2 h 1 b2 k   2 a 2 − b 2 , 2 ⋅ a 2 − b 2  where (h, k) is the point of concurrency. x2 y2 If the normals at the points a, b  and g  on the ellipse 2 + 2 = 1 are concurrent a b sec a cos eca 1 then prove that sec b secg

cos ecb cos ecg

1 = 0. 1

24.

If a, b, g  and d  are the eccentric angles of the four corner points on the ellipse x2 y2 + = 1 then prove that: (i) Σcos (a + b ) = 0 and (ii) Σsin (a + b ) = 0. a2 b2

25.

If the pole of the normal to an ellipse at P lies on the normal at Q, show that the pole of the normal at Q lies on the normal at P.

26.

Find the locus of the middle points of the chords of ellipse whose distance from 2  x2 y2   x2 y2  the centre C is constant c. Ans.:  2 + 2  = c 2  2 + 2  b  b  a a

27.

Find the locus of the midpoint of chords of the ellipse of constant length 2l.  x2 y2   x2 y2  l2  x2 y2   Ans.:  2 + 2 − 1  4 + 4  = 2 2  2 + 2  b b  a b a b  a a

28.

Show that the locus of midpoints of chords of the ellipse

29.

If the midpoint of a chord lies on a fixed line lx + my + n = 0 show that the locus  x2 y2  of the pole of the chord is the ellipse n  2 + 2 − 1 + lx + my + n = 0 . b a 

30.

Show that the locus of middle points of the chords of the ellipse that pass 2 2 h k    x −   y −  1  x2 y2  2 2 through a fixed point (h, k) is the ellipse + . = + a2 b2 h  a 2 b 2 

31.

Prove that the locus of the point of intersection of tangents to an ellipse at two points whose eccentric angles differ by a constant is an ellipse. If the sum of the eccentric angles be constant then prove that the locus is a straight line.

32.

TP and TQ are the tangents drawn to an ellipse from a point T and C is its q −f centre. Prove that the area of the quadrilateral CPTQ is ab tan where q 2 and f are the eccentric angles of P and Q.

x2 y2 + = 1, tangents at a2 b2 2 2 2 x y  the ends of which intersect on the circle x2 + y2 = a2 is a 2  2 + 2  = x 2 + y 2 . b  a

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7-78

33.

Analytical Geometry

x2 y2 + = 1 are q a2 b2 and f. Prove that the area of this parallelogram formed by the tangents at the ends of the diameters through P and Q is 4ab cosec(q − f ).

The eccentric angles of two points P and Q on the ellipse

35.

x2 y2 + = 1 pass through a fixed point (h, k). Show that a2 b2 x 2 y 2 xh yk the locus of their middle points is the ellipse 2 + 2 = 2 + 2 . a b a b If P is any point on the director circle, show that the locus of the middle points of the chord in which the polar of P cuts the ellipse is 2  x2 y2  x2 + y2 +  a 2 b 2  = a 2 + b 2 .

36.

Show that the locus of midpoints of the chords of the ellipse

37.

If the normals to an ellipse at Pi, i = 1, 2, 3, 4 are concurrent then the circle through P1, P2 and P3 meets the ellipse again in a point P4 which is the other end of the diameter through P4.

38.

Find the centre of the circle passing through the three points, on the ellipse whose eccentric angles are a, b  and g.

34.



39.

40.

41.

42.

Chords of the ellipse

x2 y2 + =1 a2 b2 2  x2 y2  x2 y2 touching the ellipse 2 + 2 = 1 is a 2 b 4 x 2 + b 2a 4 y 2 = a 4 b 4  2 + 2  . a b b  a

 b2 − a2 Ans.:  [cos a + cos b + cosg + cos(a + b + g )],  4a  a2 − b2 [cos a + cos b + cosg + cos(a + b + g )] . 4b  If ABC be a maximum triangle inscribed in an ellipse then show that the eccentric 2p angles of the vertices differ by and the normals A, B and C are concurrent. 3 The tangent and normal to the ellipse x2 + 4y2 = 2, at the point P meet the major axis in Q and R, respectively and QR = 2. Show that the eccentric angle of P is  2 cos −1   .  3 If two concentric ellipses be such that the foci of one lie on the other then prove that e12 + e22 − 1 where e1 and e2 are their eccentricities. the angle between their axes is e1e2 x2 y2 Show that the length of the focal chord of the ellipse 2 + 2 = 1 which makes a b 2ab 2 an angle q with the major axis is 2 2 . 2 2 a sin q + b cos q

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Ellipse

7-79

43.

If the normals are drawn at the extremities of a focal chord of an ellipse, prove that a line through their point of intersection parallel to the major axis will bisect the chord.

44.

If tangents from the point to the ellipse

45.

If the normal at any point P on an ellipse cuts the major axis at G, prove that the locus of the middle point of PQ is an ellipse.

46.

Show that the locus of the intersection of two normals to the ellipse 2 2 y2  x2 y2 2 2 2  x ( a − b ) ⋅ + which are perpendicular to each other is + = 1  a 2 b 2  = a2 b2 2 2 2 x y  ( a 2 + b 2 )( x 2 + y 2 )  2 + 2  . b  a x2 y2 If the angle between the diameter of any point of the ellipse 2 + 2 = 1 and a b a2 − b2 the normal at that point is q, prove that the greatest value of tanq is . 2ab

47.

x2 y2 + = 1 cut off a length equal to the a2 b2 major axis from the tangent at (a, 0), prove that T lies on a parabola.

48.

P is any point on an ellipse. Prove that the locus of the centroid G of the point P and the two foci of the ellipse is a concentric ellipse of the same eccentricity.

49.

If P, Q, R and S are conormal points on an ellipse, show that the circle passing through P and R will cut the ellipse at a point S ′ where S and S ′ are the ends of a diameter of the ellipse.

50.

Show that the locus of pole of any tangent to the ellipse with respect to the auxiliary circle is a similar concentric ellipse whose major axis is at right angles to that of the original ellipse.

51.

The normals of four points of an ellipse meet at (h, k). If two of the points lie on lx my x y + + 1 = 0, prove that the other two points lie on + − 1 = 0. a b al bm

52.

If the normals to the ellipse

53.

Prove that two straight lines through the points of intersection of an ellipse with any circle make equal angles with the axes of the ellipse.

54.

Show that the equation of a pair of straight lines which are at right angles and each of which passes through the pole of the other may be written as lx + my + n = 0 and n(mx - ny) + lm(a2 - b2) = 0. Also prove that the product of the a2 − b2 . distances of such pair of lines from the centre commonly exceeds 2

x2 y2 + = 1 at the ends of the chords lx + my = 1 a2 b2 and l1x + m1y = 1 be concurrent then show that a2ll1 = b2mm1 = −1.

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7-80

Analytical Geometry

55.

Show that the rectangle under the perpendicular drawn to the normal at a point of an ellipse from the centre and from the pole of the normal is equal to the rectangle under the focal distances of P.

56.

Prove that if P, Q, R and S are the feet of the normals to the ellipse

x2 y2 + =1 a2 b2 and the coordinates (x1, y1), (x2, y2), are the poles of PQ and RS then they are

connected by the relations x1 x22 = − a 2 and y1 y22 = − b 2 . 57.

x2 y2 + = 1 are concurrent and if a2 b2 two points lie on the line lx + my = 1, show that the other two points lie on the x y line 2 + 2 = 0. Hence show that if the feet of the two normals from a point al bm P to this ellipse are coincident then the locus of the midpoints of the chords If the normals at four points of the ellipse

2

2

 x2 y2   xy  joining the feet of the other normals is   =  2 + 2  .  ab  b  a

7.20.1  Locus of Midpoint Locus of midpoint of a series of parallel chords of the ellipse: Let (x1, y1) be the midpoint of a chord parallel to the line y = mx. Then the equation of the chord is T = S1.

C

Its slope is

(i.e.) −b 2 x1 . a 2 b1

xx1 yy1 x12 y12 + − 1 = + −1 a2 b2 a2 b2

Since this chord is parallel to y = mx, m =

−b 2 x1 −b 2 x1 (i.e.) . y = 1 a 2 y1 a2 m

−b2 x which is a straight line passing through the a2 m −b2 centre of the ellipse. If y = m1x bisect all chords parallel to y = mx then m1 = 2 . am

The locus of (x1, y1) is y =

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Ellipse

∴ mm1 =



7-81

−b2 a2

By symmetry of this result, we see that the diameter y = mx bisect all the chords parallel to y = m1x.

Definition 7.20.1 Two diameters are said to be conjugate to each other if chords parallel to one is bisected by the other. Therefore, the condition for the −b2 diameter y = mx and y = m1x to be conjugate diameters is mm1 = 2 . a 7.20.2  Property: The Eccentric Angles of the Extremities of a Pair of Semi-conjugate Diameter Differ by a Right Angle Let PCP′ and DCD′ be a pair of conjugate diameters. Let P be the points (a cos q, b sin q  ) and D be the points (a cos f, b sin f  ). Then the slope of CP b sin q is m = . a cos q P

D

C P′

D′

b sin f . a cos f Since CP and CD are semi-conjugate diameters

The slope of CD is m1 =



mm1 =

−b2 b sin q  b sin f  −b 2 ⇒ = 2 . a a cos q  a cos f  a

\ cos q cos f  = −sin q sin f (i.e.) cos q cos f + sin q sin f = 0 ⇒ cos(f − q ) = 0 ∴f − q =

p p ⇒f =q + 2 2

Therefore, the eccentric angles of a pair of semi-conjugate diameters differ by a right angle.

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7-82

Analytical Geometry

 

p  p  + q  , a sin  + q   . 2  2 

Note 7.20.1: The coordinates of D are  a cos 

(i.e.)  ( −a sin q , b cos q ) Therefore, if the coordinates of P are ( a cos q , b sin q ) then the coordinates of D are ( −a sin q , b cos q ). The coordinates of P′ are ( − a cos q , − b sin q ). The coordinates of D′ are ( a sin q , −b cos q ).

7.20.3  Property: If CP and CD are a Pair of Semi-conjugate Diameters then CD 2 + CP 2 is a Constant The coordinates of C, P and D are C(0, 0)

P ( a cos q , b sin q ) and D( − a sin q , b cos q ).

Then

CP 2 = a 2 cos 2 q + b 2 sin 2 q



CD 2 = a 2 sin 2 q + b 2 cos 2 q



∴ CP 2 + CD 2 = a 2 (cos 2 q + sin 2 q ) + b 2 (sin 2 q + cos 2 q ) = a 2 + b 2 = constant



7.20.4  Property: The Tangents at the Extremities of a Pair of Conjugate Diameters of an Ellipse Encloses a Parallelogram Whose Area Is Constant F

D

G

P

C P′

E

D′

H

Let PCP′ and DCD′ be a pair of conjugate diameters. Let P be the point  p  p  ( a cos q , b sin q ) . Then D is the point  a cos  + q  , b sin  + q   .     2 2 

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Ellipse

7-83

(i.e.)  ( −a sin q , b cos q ). x y cos q + sin q = 1. a b

The equation of the tangent at P is

The slope of the tangent at P is −b cos q . a sin q −b cos q . The slope of CD is a sin q Since the two slopes are equal, the tangents at P is parallel to DCD′. Similarly, we can show that the tangent at P′ is parallel to DCD′. Therefore, the tangent at P and P′ are parallel. Similarly, the tangent D and D′ are parallel. Hence, the tangents at P, P′, D, D′ from a parallelogram EFGH. The area of the parallelogram EFGH = 4 × Area of parallelogram CPFD = 4 × 2 area of DCPD = 8 area of DCPD

1 = 8 × [a cos q (b cos q ) − a sin q ( − b sin q )] 2



= 4 ab(cos 2 q + sin 2 q )



= 4ab which is a constant.

7.20.5  Property: The Product of the Focal Distances of a Point on an Ellipse Is Equal to the Square of the Semi-diameter Which Is Conjugate to the Diameter Through the Point P

M′ X′

S′

C

S

N

M X

x2 y2 + = 1 . Let P be any point on the a2 b2 ellipse and draw MPM′ perpendicular to the directrix.

Let S and S′ be the foci of ellipse

Then,

SP = e ⇒ SP = ePM = e ⋅ NX = e ⋅ [CX − CN ] PM

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Analytical Geometry



a  = e  − a cos q  = a − ae cos q e 



S ′ P = e ⋅ PM ′ = e ⋅ NX ′ = e[CX ′ + CN ]



a  = e  + a cos q  = a + ae cos q e  

SP ⋅ S ′P = ( a − ae cos q )( a + ae cos q ) = a 2 − a 2 e 2 cos 2 q = a 2 − ( a 2 − b 2 ) cos 2 q



= a 2 (1 − cos 2 q ) + b 2 cos 2 q = a 2 sin 2 q + b 2 cos 2 q = CD 2

7.20.6  Property: If PCP ′ and DCD ′ are Conjugate Diameter then They are also Conjugate Lines We know that the polar of a point and the chord of contact of tangents from it to the ellipse are the same. Therefore, the pole of the diameter PCP′ will be the point of intersection of the tangents at P and P′ which are parallel. Therefore, the pole of PCP′ lies at infinity on the conjugate diameter DCD′. Hence, PCP′ and DCD′ are conjugate lines.

Note 7.20.2: Conjugate diameter is a special case of conjugate lines.

7.21  EQUI-CONJUGATE DIAMETERS Definition 7.21.1 Two diameters of an ellipse are said to be equi conjugate diameters if they are of equal length.

7.21.1  Property: Equi-conjugate Diameters of an Ellipse Lie along the Diagonals of the Rectangle Formed by the Tangent at the Ends of its Axes x2 y2 + = 1. a2 b2 Let the coordinates of P be ( a cos q , b sin q ). Then the coordinates of D are (−a sin q, b cos q ). C is the point C(0, 0). Let PCP′ and DCD′ be two conjugate diameters of the ellipse



CP 2 = a 2 cos 2 q + b 2 sin 2 q CD 2 = a 2 sin 2 q + b 2 cos 2 q

If CP and CD are equi-conjugate diameters then CP2 = CD2.

a 2 cos 2 q + b 2 sin 2 q = a 2 sin 2 q + b 2 cos 2 q .

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Ellipse



( a 2 − b 2 ) sin 2 q = ( a 2 − b 2 ) cos 2 q



∴ tan 2 q = 1 or tan q = ±1 ∴q =

When q =

p 3p or 4 4

p , the equation of the diameter is 4 b sin q b (i.e.) y = x y= a cos q a

The equation of CD is y =

(i.e.) y =

7-85

(7.101)

b cos q x − a sin q −b x a

(7.102)

3p b , equations of these two conjugate diameters are y = ± x. 4 a b Therefore, the equi-conjugate diameters are y = ± x which are the equaa tions of the diagonals formed by the tangents at the four vertices of the ellipse.

When q =

ILLUSTRATIVE EXAMPLES BASED on conjugate diameters Example 7.50 Show that the locus of the point of intersection of tangents at the extremix2 y2 ties of a pair of conjugate diameters of the ellipse 2 + 2 = 1 is the ellipse a b x2 y2 . + = 2 a2 b2

Solution

x2 y2 Let PCP′ and DCD′ be a pair of conjugate diameters of the ellipse 2 + 2 = 1. a b Let P be the point (a cos q b sin q  ). Then D′ is the point

 p p     a cos q + 2  , b sin q + 2   (i.e.)  (-a sin q, b cos q  )

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7-86

Analytical Geometry

D

P

C

P′

D′

Let (x1, y1) be the point of intersection of the tangents at P and D. The equations of the tangents at P and D are x y cos q + sin q = 1 a b x y − sin q + cos q = 1 a b Since these two tangents meet at (x1, y1), x1 y cos q + 1 sin q = 1  a b and − x1 y sin q + 1 cos q = 1 a b Squaring and adding from equations (7.103) and (7.104), we get Therefore, the locus of (x1, y1) is

(7.103)

(7.104) x12 y12 + = 2. a2 b2

x2 y2 + = 2. a2 b2

Example 7.51 If P and D are the extremities of a pair of conjugate diameter of the ellipse x2 y2 x2 y2 1 + = 1 + = . show that the locus of the midpoint of PD is a2 b2 a2 b2 2

Solution Let P be the point (a cos q, b sin q ). Then the coordinates of D are (-a sin q, b cos q ). Let (x1, y1) be the midpoint of PD. b sin q + b cos q a cos q − a sin q Then x1 = , y1 = 2 2 2 x1 (i.e.)  (7.105) = cos q − sin q  a 2 y1 (7.106) = sin q + cos q  b

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Ellipse

7-87

Squaring and adding from equations (7.105) and (7.106), we get 4 x12 4 y12 + 2 =2 a2 b x12 y12 1 (i.e.)  2 + 2 = 2 a b



Therefore, the locus of (x1, y1) is

x2 y2 1 + = which is a concentric ellipse. a2 b2 2

Example 7.52 x2 y2 If CP and CD are two conjugate semi-diameters of an ellipse 2 + 2 = 1 a b x2 y2 1 then prove that the line PD touches the ellipse 2 + 2 = . 2 a b

Solution

p Let the eccentric angle of P be q.  Then the eccentric angle of D is q + . The 2 equation of the chord PD is  q  p q   q − q +   q +q + q +q +   x  2 2  + y sin  2  = cos  cos    b   a 2 2 2            

(i.e) (i.e)

(i.e.) 

p p 1 p y x   cos q +  + sin q +  = cos =   4 b 4 4 a 2 x y p p   cos q +  + sin q +  = 1 b    4 4  a    2 2

x y cos f + sin f = 1 A B

where A =

a 2

, B=

b 2

and q +

p =f. 4

This straight line touches the ellipse (i.e.) 

x2 y2 + = 1. A2 B 2

2x2 2 y2 x2 y2 1 or 1 + = + = a2 b2 a2 b2 2

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7-88

Analytical Geometry

Example 7.53 Find the condition that the two straight lines represented by Ax2 + 2Hxy + x2 y2 By2 = 0 may be a pair of conjugate diameters of the ellipse 2 + 2 = 1. a b

Solution Let the two straight lines represented by Ax2 + 2Hxy + By2 = 0 be y = m1x −2 H A and m1m2 = . and y = m2x. Then m1 + m2 = B B b2 The condition for the lines to be conjugate diameters is m1m2 = − 2 . a A −b2 2 2 (i.e.)  = 2 ⇒ a A+ b B = 0 B a This is the required condition.

Example 7.54 If P and D be the ends of conjugate semi-diameters of the ellipse then show that the locus of the foot of the perpendicular from the centre on the line PD is 2(x2 + y2)2 = a2x2 + b2y2.

Solution

p Let the eccentric angle of P be q.  Then the eccentric angle of D is q + . 2 The equation of PD is

x p y p 1    cos q +  + sin q +  =   a 4 b 4 2

(7.107)

The equation of the line perpendicular to this and passing through the centre (0, 0) is

x p y p   sin q +  − cos q +  = 0    b 4 a 4

(7.108)

Let (x1, y1) be the foot of the perpendicular from (0, 0) on PD. Then (x1, y1) lies on the above two lines.

x1 1 p y p    cos  q +  + 1 sin q +  =     a b 4 4 2

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(7.109)

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Ellipse



x1 p y p   sin q +  − 1 cos q +  = 0    b 4 a 4

7-89

(7.110)

p  p Solving for cos q +  and sin q +  , we get   4  4



p p   cos q +  sin q +    4 4 = = ax1 by1

1 a x + b 2 y12 2

2 1

p  p  Substituting for cos q +  and sin q +  in equation (7.107), we get   4 4 x12 + y12



x a +b y 2 1

2

2

2 1

=

1 2

(i.e.)  2( x12 + y12 )2 = a 2 x12 + b 2 y12 Therefore, the locus of (x1, y1) is 2(x2 + y2) = a2x2 + b2y2.

Example 7.55

x2 y2 + = 1 . If the a2 b2 circles on CP and CD as diameters intersect in R then prove that the locus of the point R is 2(x2 + y2)2 = a2x2 + b2y2.

CP and CD are semi-conjugate diameters of the ellipse

Solution Let P be the point (a cos q, b sin q ). Then D is the point (−a sin q, b cos q ). C is the point (0, 0). The equations of the circles on CP and CD as diameters are x(x - a cos q) + y(y - b sin q ) = 0 and x(x + a sin q ) + y(y - b cos q ) = 0. (i.e.)  x2 + y2 = ax cos q  + by sin q and x2 + y2 = −ax sin q + by cos q. Let (x1, y1) be a point of intersection of these two circles. Then

x12 + y12 = ax1 cos q + by1 sin q 

(7.111)



x12 + y12 = − ax1 sin q + by1 cos q 

(7.112)

By squaring and adding equations (7.111) and (7.112), we get ( x12 + y12 )2 = a 2 x12 + b 2 y12 . Therefore, the locus of (x1, y1) is (x2 + y2)2 = a2 x2 + b2 y2.

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7-90

Analytical Geometry

Example 7.56

x2 y2 x2 y2 + = 1 and + = 1 be the a2 b2 a2 b2 a2 b2 points of conjugate diameters of the former prove that 2 + 2 = 2. A B

If the points of intersection of the ellipses

Solution Any conic passing through the point of intersection of the ellipses x2 y2 + =1 a2 b2

(7.113)

x2 y2 + = 1 A2 B 2

(7.114)

 x2 y2   x2 y2   a 2 + b 2 − 1 + l  A2 + B 2 − 1 = 0 

(7.115)

and is

where l = −1, equation (7.118) reduces to  x2 y2   x2 y2   a 2 + b 2  −  A2 + B 2  = 0



1 1  1  1 (i.e.)  x 2  2 − 2  + y 2  2 + 2  = 0 a b A  B 

(7.116)

This being a homogeneous equation of second degree in x and y represents a pair of straight lines, that is, equation (7.116) represents a pair of straight lines b2 passing through the origin. m1m2 = − 2 . a

(i.e.)  or

1 1 − 2 2 2 a 2 A2 = − b ⇒ 1 − a = 1 + b 2 2 1 1 a A B2 − b2 B 2

a2 b2 + = 2 which is required condition. A2 B 2

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Ellipse

7-91

Example 7.57 If a  and b  be the angles subtended by the major axis of an ellipse at the ­extremities of a pair of conjugate diameters then show that cos 2 a  + cos 2 b  is a constant.

Solution x2 y2 + =1. a2 b2 Let P be the point (a cos a, b sin b ).  p    a cos(a + p ), b sin  a + 2   .   Let equation of the ellipse be

The slope of AP is m1 = The slope of A′P is m2 =

is

the

point

b sin a . a(cos a + 1) P

b

a

A′ (−a, 0)

tan a = ±

A (a, 0)

( m1 − m2 ) 1 + m1m2

b sin a b sin a − a(cos a − 1) a(cos a + 1) =± b 2 sin 2 a 1+ 2 a (cos 2 a − 1) =±



D

b sin a . a(cos a − 1)

D



Then

tan a = ±

ab sin a (cos a + 1 − cos a + 1) a 2 (cos 2 a − 1) + b 2 sin 2 a 2ab sin a ( a 2 − b 2 ) sin 2 a

\ cot 2 a =

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( a 2 − b 2 )2 sin 2 a  4a2 b2

(7.117)

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7-92

Analytical Geometry

Changing a into a +

p , 2 ∴ cot 2 b =



( a 2 − b 2 )2 cos 2 a  4a2 b2

(7.118)

Adding equations (7.117) and (7.118), we get

cot 2 a + cot 2 b =

( a2 − b2 )2 = a constant. 4a2 b2

Example 7.58 If x cos  a + y sin  a = p is a chord joining the ends P and D of conjugate semi-diameters of the ellipse then prove that a2 cos2 a + b2 sin2 a  = 2p2.

Solution x2 y2 + = 1 . Let PCP′ and DCD′ be a pair a2 b2 of conjugate diameters. Let P be the point (a cos q, b sin q ) then D is the point  p p     a cos q + 2  , b sin q + 2  .  

Let the equation of the ellipse be

D

P

C P′

D′

The equation of PD is

x p y p 1    cos q +  + sin q +  =   a 4 b 4 2

(7.119)

However, the equation of PD is given as

x cos  a + y sin  a = p

(7.120)

Equations (7.119) and (7.120) represent the same line. Identifying equations (7.119) and (7.120), we get

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7-93

Ellipse

p p   cos  q +  sin q +     4 4 = = a cos a b sin a

1 2p

p  a cos a  cos q +  =  4 2p p  b sin a  sin q +  =  4 2p (i.e.)  a2cos 2 a  + b2sin2 a  = 2p2

Example 7.59 x2 y2 + = 1. A tangent is a2 b2 drawn parallel to PD meeting CP and CD in R and S respectively. Prove that x2 y2 R and S lie on the ellipse 2 + 2 = 2. a b

CP and CD are conjugate diameters of the ellipse

Solution Let CP and CD be a pair of conjugate diameters of the ellipse x2 y2 + =1 a2 b2



S p q+ 2

(7.121) R

D

P C

Let P be the point (a cos q, b sin q ). Then D is the point (−a sin q, b cos q ). Slope of PD is

m=

b(sin q − cos q ) b(tan q − 1)  = a(cosq + sin q ) a(tan q + 1)

(7.122)

Let the equation of the tangent parallel to PD be y = mx + a 2 m2 + b 2 .

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7-94

Analytical Geometry

Let R be the point (h, k). Since (h, k) lies on this tangent, ( k − mh) = a 2 m2 + b 2 



(7.123)

b sin q x. a cos q b sin q Since this passes through (h, k), k = h. a cos q

In addition, the equation of CP is y =

ak = tanq bh Substituting in equation (7.122), we get \



m=



b( ak − bh) a( ak + bh)

(7.124)

Eliminating m from equations (7.123) and (7.124), b ak − bh k −h ⋅ = a ak + bh

a2 b2 a2

2

 ak − bh  2   +b ak + bh 

a 2 k 2 + b 2 h2 = ab 2( a 2 k 2 + b 2 h2 )



( a 2 k 2 + b 2 h2 ) = ab 2 (i.e.) ( a 2 k 2 + b 2 h2 ) = 2a 2 b 2 Dividing by a2b2,

k 2 h2 x2 y2 + 2 = 2. The locus of (h, k) is 2 + 2 = 2. Similarly, 2 b a a b

the point S also lies on the above ellipse.

Example 7.60 x2 y2 2 2 2 2 + 2 = 1 cuts the circle x + y = a + b in P and 2 a b Q. Prove that CP and CQ are along conjugates semi-diameters of the ellipse where C is the centre of the circle. A tangent to the ellipse

Solution The equation of the ellipse is

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x2 y2 + = 1 a2 b2

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Ellipse

Q

‘q ’

7-95

P

C

The equation of the circle is x2 + y2 = a2 + b2.

(7.125)

The equation of the tangent at q on the ellipse is

x y cos q + sin q = 1 a b

(7.126)

This meets the circle in P and Q. The combined equation CP and CQ is got by homogenization of equation (7.125) with the help of equation (7.126), 2

 x cos q y sin q  x 2 + y 2 = ( a2 + b2 )  +   a b   a2 + b2   2( a 2 + b 2 )   a2 − b2  x 2 1 − cos 2 q  + xy  cos q sin q  + y 2 1 − sin 2 q  = 0 2 2 ab a b         a 2 − ( a 2 + b 2 ) cos 2 q b2 m1m2 =  ⋅ 2  2 2 2 2 ( ) sin q a b − a + b   \ CP and CQ are conjugate semi-diameters of the ellipse.

Example 7.61 Prove that the acute angle between two conjugate diameters is least when they are of equal length.

Solution Let PCP′ and DCD′ be the conjugate diameters.    2  2   (CP − CD )2 = CP + CD − 2CP ⋅ CD     2CP ⋅ CD = ( a 2 + b 2 ) − (CP − CD )2   2  2 ∵ CP + CD = a 2 + b 2

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(7.127)

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7-96

Analytical Geometry

P

F

D

E

D′

C

G

H

P′

Now area of paralleogram CPFD = ab sin PCD .   ab sin PCD = CP ⋅ CD 

(7.128)

From equations (7.127) and (7.128),   2ab = a 2 + b 2 − (CP − CD )2 sin PCD sin PCD =

2ab   a + b − (CP − CD )2 2

2

  RHS is least when the denominator is the largest. This happens when CP = CD. Therefore, the acute angle between the diameters is minimum when the conjugate diameters are of equal length and the least acute angle is given by  2ab  q = sin −1  2 .  a + b 2 

Example 7.62 Find the locus of the point of intersection of normals at two points on an ­ellipse which are extremities of conjugate diameters.

Solution Let the equation of the ellipse be x2 y2 + =1 a2 b2

D

q + p /2

(7.129)

P

′q ′ C



M07_ANALYTICALGEOMETRY_ISBN_CH07.indd 96

P′

D′

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Ellipse

7-97

Let P and D be the extremities of a pair of conjugate diameters of the ellipse (7.129). Let P and D be the points P(a cos q, b sin q ) and D(−a sin q, b cos q ). The equations of the normal at P and D are ax by − = a2 − b2 cos q sin q ax by − − = a2 − b2  sin q cos q

(i.e.) 

ax by + = −( a 2 − b 2 )  sin q cos q

(7.130) (7.131)

Solving equations (7.130) and (7.131), we get,

cos q =

a2 x 2 + b2 y 2  ( ax − by )( a 2 − b 2 )

(7.132)



sin q =

a2 x 2 + b2 y 2  −( ax + by )( a 2 − b 2 )

(7.133)

Squaring and adding, we get 1=

 ( a2 x 2 + b2 y 2 )2  1 1 +  2 2 2 2 2 (a − b )  ( ax − by ) ( ax + by )  2

 a2 x 2 + b2 y 2  2( a 2 x 2 + b 2 y 2 ) = 1=  a2 x 2 − b2 y 2  a 2 − b 2  Therefore, the locus of the point of intersection of these two normals is (a2x2 + b2y2)3 = (a2 - b2)2 (a2x2 - b2y2)2.

Example 7.63

x2 y2 a2 b2 + 2 = 1 and 2 + 2 = 1 be 2 a b a b at the extremities of the conjugate diameters of the former then prove that x2 y2 + = 2. a2 b2 If the point of intersection of the ellipses

Solution The given ellipses are

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x2 y2 + =1 a2 b2

(7.134)

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7-98

Analytical Geometry

x2 y2 + =1 a2 b2



(7.135)

Solving equations (7.134) and (7.135) we get their point of intersections.  1 1  1   1 Equation (7.134) – (7.135) gives x 2  2 − 2  + y 2  2 − 2  = 0. a a  b b  This is a pair of straight lines passing through the origin. If y = mx is one of  1 1  1   1 the lines then x 2  2 − 2  + m2 x 2  2 − 2  = 0 a a  a b 

(i.e.) 

 1 1 1 1  − 2 + m2  2 − 2  = 0  2 a a b b 

(7.136)

This is a quadratic equation in m. If m1 and m2 are the slopes of the two straight lines through the origin then 1 1 − 2 2 m1m2 = a a  1 1 − 2 2 b b



(7.137)

If m1 and m2 are the slopes of the pair of conjugate diameters then m1m2 =



−b2 a2

(7.138)

From equations (7.137) and (7.138), we get 1 1 − a2 a 2 = −b 1 1 a2 − 2 2 b b  1 1  1   1 (i.e.) a 2  2 − 2  = −b 2  2 − 2  a a  b b  1− ∴

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a2 b2 = −1 + 2 2 a b

a2 b2 + =2 a2 b2

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Ellipse

7-99

Example 7.64 Let P and Q be the extremities of two conjugate diameters of the ellipse x2 y2 + 2 = 1 and S be the focus. Then prove that PQ2 − (SP − SQ)2 = 2b2. 2 a b

Solution Let S be (ae, 0) and P be (a cos q, b sin q ). Then SP = a - aecos q, SQ = a + aesin q. PQ 2 = ( a cosq + a sin q )2 + (b sin q − b cos q )2



= a 2 + 2a2 cos q sin q − 2b 2 cos q sin q + b2 = a 2 + b 2 + 2 cosq sinq ( a 2 − b 2 )



= a 2 + b 2 + 2( a 2 − b 2 ) cosq sinq ( SP − SQ )2 = [( a − ae cosq ) − ( a + ae sinq )]2



= a 2 e 2 (cosq + sinq )2 = a 2 e 2 (1 + 2 sinq cosq ) PQ 2 − ( SP − SQ )2 = a 2 + b 2 + 2( a 2 − b 2 ) sin q cos q − ( a 2 − b 2 ) − ( a 2 − b 2 ) cos q sin q



= a2 + b2 − (a2 − b2 ) = 2b 2

Example 7.65 x2 y2 + = 1, a2 b2 prove that the locus of the orthocentre of ∆PCD is 2(b2y2 + a2x2)3 = ( a2 − b2 )2 ( a2 x 2 − b2 y 2 )2 .

If CP and CD are semi-conjugate diameters of the ellipse

Solution P

D

C

c

H

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7-100

Analytical Geometry

Let P be the point (a sin q, b cos q ). Then D is (-a sin q, b cos q ). Tangent at P is parallel to CD. Tangent at D is parallel to CP. Therefore, the altitudes through P and D are the normals at P and D, respectively. Let (x1, y1) be the orthocentre. The equation of the normal at P is ax by − = a2 − b2 cos q sin q



(7.139)

The equation of the normal at Q is − ax by − = a2 − b2  sin q sin q



(7.140)

Solving equations (7.139) and (7.140) we get the coordinates of the ­orthocentre. 1 1 a2 − b2 = = 2 2 ( ax − by ) cos q −( ax + by ) sin q a x + b 2 y 2 ∴cos q =

a2 x 2 + b2 y 2 a2 x 2 + b2 y 2 q = , sin + ( a 2 − b 2 )( ax − by ) −( a 2 − b 2 )( ax + by )

Squaring and adding, we get

1=



 1 1 ( a2 x 2 + b2 y 2 )2  +  2 2 2 2 2 (a − b )  ( ax − by ) ( ax + by ) 

2( a 2 x 2 + b 2 y 2 ) ( a2 − b2 )2 = 2 2 2 2 2 2 ( a x − b2 y 2 )2 (a x + b y ) Therefore, the locus of the orthocentre is 2( a 2 x 2 + b 2 y 2 )3 = ( a 2 − b 2 )2 ( a 2 x 2 − b 2 y 2 )2 . ∴

2

Exercises 1. 2.

Let CP and CQ be a pair of conjugate diameters of an ellipse and let the tangents at P and Q meet at R. Show that CR and PQ bisect each other. x2 y2 Find the condition that for the diameters of 2 + 2 = 1 through its points of a b intersection with the line lx + my + n = 0 to be conjugate.



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Ans.: l2 + m2 = a2l2 + b2m2

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Ellipse

3.

4. 5.

6. 7. 8.

9. 10.

11. 12. 13.

7-101

Prove that b 2 x 2 + 2hxy − a 2 y 2 = 0 represents conjugate diameters of the x2 y2 ellipse 2 + 2 = 1 for all values of h. a b Prove that a 2 x 2 + 2hxy − b 2 y 2 = 0 represents conjugate diameters of the ellipse ax2 + by2 = 1 for all values of h. Find the coordinates of the ends of the diameter of the ellipse 16x2 + 25y2 = 5 5 2 2 ), ( −2 2 , ) 400 which is conjugate to 5y = 4x. Ans.: (2 2 , − 2 2 Find the length of semi-diameter conjugate to the diameter whose equation is y = 3x. Through the foci of an ellipse, perpendiculars are drawn to a pair of conjugate diameters. Prove that they meet on a concentric ellipse. x2 y2 A diameter of the ellipse 2 + 2 = 1 meets one latus rectum in P and the a b conjugate diameter meets the other latus rectum in Q. Prove that PQ touches x2 y2 + = e2 . a2 b2 If PP′ is a diameter and Q is any point on the ellipse, prove that QP and QP′ are parallel to a pair of conjugate diameters of the ellipse. If a + b  = g  (a constant) then prove that the tangents at a  and b  on the ellipse x2 y2 + = 1. intersect on the diameter through g. a2 b2 Show that the line joining the extremities of any two diameters of an ellipse which are at right angles to one another will always touch a fixed circle. Show that the sum of the reciprocals of the square of any two diameters of an ellipse which are at right angles to one another is a constant. x2 y2 P and Q are extremities of two conjugate diameters of the ellipse 2 + 2 = 1. a b and S is a focus. Prove that PQ2 + (SP - SQ)2 = 2b2.

14.

If the distance between the two foci of an ellipse subtends angles 2a and 2b  at the ends of a pair of conjugate diameters. Show that tan2a + tan2b  is a constant.

15.

Show that the sum of the squares of the normal at the extremities of conjugate semi-diameters and terminated by major axis is a2(1 - e2)(2 - e2).

16.

If P and Q are two points on an ellipse such that CP is conjugate to the normal at Q, prove that CQ is conjugate to the normal at P.

17.

Two conjugate diameters of the ellipse

18.

x2 y2 + = 1 centre at C meet the tangent a2 b2 at any point P is E and F. Prove that PE ⋅ PF = CD2. x2 y2 If CP and CD are conjugate semi-diameters of the ellipse 2 + 2 = 1, the a b normal at P cuts the major axis at G and the line DC in F then prove that PG : CD = b : a.

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7-102

19.

Analytical Geometry

x2 y2 + = 1 cuts the diameter a2 b2 CD conjugate to P in Q. Prove that the equation of the locus of Q is

The normal at a variable point P of an ellipse 2

a2 b2  a2 − b2  + = . x 2 y 2  x 2 + y 2  20.

Show that for a parallelogram inscribed in an ellipse, the sum of the squares of the sides is constant.

21.

Show that the maximum value of the smaller of two angles between two conjugate p 2ab  diameters of an ellipse is and the minimum value of this angle is tan −1  2   a − b2  2 where a and b are its semi-major and semi-minor axes, respectively. x2 y2 If PCP′ and DCD′ are two conjugate diameters of the ellipse 2 + 2 = 1. and a b Q is any point on the circle x2 + y2 = c2 then prove that PQ2 + DQ2 + P2Q2 + PQ2 = 2(a2 + b2 + 2c2). x2 y2 Two conjugate diameters of the ellipse 2 + 2 = 1 cut the circle x2 + y2 = r2 at a b P and Q. Show that the locus of the midpoint of PQ is a2[(x2 + y2)2 - r2x2] + b2[(x2 + y2)2 - r2y2] = 0.

22.

23.

24.

In an ellipse whose semi-axes are a and b, prove that the acute-angle between −1  2ab  . two conjugate diameters cannot be less than sin  2  a + b 2 

25.

If CP and CD are conjugate diameters of an ellipse show that 4(CP2 - CD2) = (SP - S′P)2 - (SD - S′D)2.

26.

Two conjugate semi-diameters of an ellipse are inclined at angles a  and b  to the major axis. Show that their lengths c and d are connecting the relation c2 sin 2a  + d2 sin 2b  = 0.

27.

Find the condition for the lines l 1x + m1 y = 0 and l2 x + m2 y = 0 to be conjugate x2 y2 diameters of 2 + 2 = 1.  Ans.: a2ll1 + b2mm1 = 0 a b

28.

Show that ax2 + 2hxy − by2 = 0 represents conjugate diameters of the ellipse ax2 + by2 = 1 for all values of a.

29.

Prove that ax2 + 2hxy − by2 = 0 represents conjugate diameters of the ellipse ax2 + by2 = 1 for all values of h. x2 y2 CP and CQ are conjugate semi-diameters of the ellipse 2 + 2 = 1. A tangent a b parallel to PQ meets CP and CQ in R and S, respectively. Show that R and S lie x2 y2 on the ellipse 2 + 2 = 2. a b

30.

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Ellipse

31. 32.

7-103

If two conjugate diameters CP and CQ of an ellipse cut the director circle in L and M, prove that LM touches the ellipse. x2 y2 Two conjugate diameters of the ellipse 2 + 2 = 1 cuts the circle x2 + y2 = r at a b P and Q. Show that the locus of the midpoint of PQ is a 2 [( x 2 + y 2 )2 − r 2 x 2 ] + b 2 [( x 2 + y 2 )2 − r 2 y

a 2 [( x 2 + y 2 )2 − r 2 x 2 ] + b 2 [( x 2 + y 2 )2 − r 2 y 2 ] = 0.

x2 y2 + = 1 are a2 b2 a  and b. Prove that the area of the parallelogram formed by the tangents at the 4ab ends of the diameters through P and Q is and hence show that it is sin(a − b ) least when P and Q are the extremities of a pair of conjugate diameters.

33.

The eccentric angles of two points P and Q on the ellipse

34.

Let PCP′ be a diameter of the ellipse

35.

If two conjugate diameters CP and CQ of an ellipse cut the director circle in L and M, prove that LM touches the ellipse.

36.

In an ellipse, a pair of conjugate diameters is produced to meet a directrix. Show that the orthocentre of the triangles so formed is a focus.

37.

Through a fixed point P, a pair of lines is drawn parallel to a variable pair of conjugate diameters of a given ellipse. The lines meet the principal axes in Q and R, respectively. Show that the midpoint of QR lies on a fixed line.

38.

Perpendiculars PM and PN are drawn from any point P of an ellipse on the equi-conjugate diameter of the ellipse. Prove that the perpendiculars from P to its polar bisect MN.

39.

In the ellipse 3x2 + 7y2 = 21, find the equations of the equi-conjugate diameters and their lengths. Ans.: 3 x ± 7 y = 0. x2 y2 Prove that the tangents to the ellipse 2 + 2 = 1 at the points whose eccentric a b p angles are q and − q meet on one of the equi-conjugate diameters. 2 From a point on one of the equi-conjugate diameters of an ellipse tangents are drawn to the ellipse. Show that the sum of the eccentric angles of the point of p contact is an odd multiple of . 2 x2 y2 Tangents are drawn from any point on the ellipse 2 + 2 = 1 to the circle

40.

41.

42.

x2 y2 + = 1. If the normal at P meets the a2 b2 x2 b2 y 2 ordinate at P′ in T, show that the locus of T is 2 + = 1. ( 2a 2 − b 2 ) a

a

b

x2 + y2 = r2. Prove that the chords of contact are tangents to the ellipse a2x2 + 1 1 1 b2y2 = r4. If 4 = 2 − 2 , prove that the line and the centre to the points of r a b contact with the circle are conjugate diameters of the second ellipse.

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7-104

Analytical Geometry

43.

Any tangent to an ellipse meets the director circle in P and D. Prove that CP and CD are in the directions of conjugate diameters of the ellipse.

44.

If CP is conjugate to the normal at Q, prove that CQ is conjugate to the normal at P.

45.

Prove that the straight lines joining the centre to the intersection of the straight line y = mx +

a2 m2 + b2 with the ellipse are conjugate diameters. 2

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Chapter

8

Hyperbola

8.1  DEFINITION A hyperbola is defined as the locus of a point that moves in a plane such that its distance from a fixed point is always e times (e > 1) its distance from a fixed line. The fixed point is called the focus of the hyperbola. The fixed straight line is called the directrix and the constant e is called the eccentricity of the hyperbola.

8.2  STANDARD EQUATION y B P x A′

C

A S N B′

x

l

Let S be the focus and the line l be the directrix. Draw SX perpendicular to the directrix. Divide SX internally and externally in the ratio e : 1 (e > 1). Let SA SA′ = e and A and A′ be the point of division. Since = e, the points A AX A ′X and A′ lie on the curve. Let AA′ = 2a and C be its middle point. SA = e ⋅ AX

(8.1)

SA′ = e ⋅ A′X Adding equations (8.1) and (8.2), we get

(8.2)





SA + SA′ = e(AX + A′X)

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8-2

Analytical Geometry



(CS - CA) + (CS + CA′) = eAA′ 2CS = e ⋅ 2a \ CS = ae

Subtracting equation (8.1) from equation (8.2), we get

SA′ - SA = e(A′X - AX) AA′ = e ⋅ 2CX 2a = e ⋅ 2CX a ∴ CX = e

Take CS as the x-axis and CY perpendicular to CX as the y-axis. Then, the coordinates of S are (ae, 0). Let P(x, y) be any point on the curve. Draw PM perpendicular to the directrix and PN perpendicular to x-axis. SP = e. From the focus directrix property of hyperbola, PM 2 2 2 SP = e ⋅ PM

⇒ (x - ae)2 + y2 = e2 ⋅ NX 2 = e2(CN - CX)2 a  = e x−   e

2

2



x2 - 2aex + a2e2 + y2 = e2 x2 - 2aex + a2



x2 (e2 - 1) - y2 = a2 (e2 - 1)

Dividing by a2 (e2 - 1), we get x2 y2 − =1 a 2 a 2 (e 2 − 1)



x2 y2 − = 1 where b2 = a2 (e2 - 1) a2 b2 This is called the standard equation of hyperbola. (i.e.) 

Note 8.2.1: 1. The curve meets the x-axis at points (a, 0) and (-a, 0). 2. When x = 0, y2 = -a2. Therefore, the curve meets the y-axis only at imaginary points, that is, there are no real points of intersection of the curve and y-axis. 3. If (x, y) is a point on the curve, (x, -y) and (-x, y) are also points on the curve. This shows that the curve is symmetrical about both the axes.

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8-3

Hyperbola

4. For any value of y, there are two values of x; as y increases, x increases and when y → ∞, x also → ∞. The curve consists of two symmetrical branches, each extending to infinity in both the directions. 5. AA′ is called the transverse axis and its length is 2a. 6. BB′ is called the conjugate axis and its length is 2b. 7. A hyperbola in which a = b is called a rectangular hyperbola. Its equaa2 + b2 = 2. a2

tion is x2 - y2 = a2. Its eccentricity is e =

8. The double ordinate through the focus S is called latus rectum and its 2b 2 . length is a 9. There is a second focus S′ and a second directrix l′ to the hyperbola.

8.3  IMPORTANT PROPERTY OF HYPERBOLA M

X′

A′

y

C

M P

X

A S N

x

The difference of the focal distances of any point on the hyperbola is equal to the length of transverse axis.

SP = ePM; S′P = ePM′

 a S ′P − SP = e( PM ′ − PM ) = e ⋅ MM ′ = eXX ′ = e(CX + CX ′ ) = e  2 ⋅  = 2a.  e

8.4  EQUATION OF HYPERBOLA IN PARAMETRIC FORM x2 y2 − = 1 for all values of q, a2 b2 q is called a parameter and is denoted by ‘q ’. The parametric equations of hyperbola are x = a sec q, y = b tan q.

(a sec q, b tan q ) is a point on the hyperbola

8.5  RECTANGULAR HYPERBOLA A hyperbola in which b = a is called a rectangular hyperbola. The standard equation of the rectangular hyperbola is x2 - y2 = a2.

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8-4

Analytical Geometry

8.6  CONJUGATE HYPERBOLA y S

C

x

S′

The foci are S(ae, 0) and S′(-ae, 0) and the equations of the directrices are a x = ± . By the symmetry of the hyperbola, if we take the transverse axis as e the y-axis and the conjugate axis as x-axis, then the equation of the hyperbola y2 x2 x2 y2 is 2 − 2 = 1 (i.e.) 2 − 2 = −1. b a a b This hyperbola is called the conjugate hyperbola. Here, the coordinates of the foci are S(0, be) and S′(0, -be). The equations of the directrices are b y=± . e The length of the transverse axis is 2b. The length of the conjugate axis is 2a. 2a 2 . The length of the latus rectum is b The following are some of the standard results of the hyperbola whose equation is

x 2 y2 − =1: a 2 b2

xx1 yy1 − 2 = 1. a2 b 2 2 The equation of the normal at ( x1 , y1 ) is a x + b y = a 2 + b 2 . x1 y1 The equation of the chord of contact of tangents from ( x1 , y1 ) is xx1 yy1 − 2 = 1. a2 b xx yy The polar of ( x1 , y1 ) is 21 − 21 = 1 . a b The condition that the straight line y = mx + c is a tangent to the hyperbola is c 2 = a 2 m2 − b 2 and y = mx + a 2 m2 − b 2 is the equation of a tangent. The equation of the chord of the hyperbola having ( x1 , y1 ) as the midpoint xx yy x2 y2 xx yy x2 y2 is T = S1 (i.e.) 21 − 21 − 1 = 12 − 12 − 1 or 21 − 21 = 12 − 12 . a b a b a b a b

1. The equation of the tangent at ( x1 , y1 ) is 2. 3.

4. 5. 6.

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8-5

Hyperbola

7. The equation of the pair of tangents from ( x1 , y1 ) is T 2 = SS1 2

2 2 2 2 (i.e.)   xx1 − yy1 − 1 =  x − y − 1  x1 − y1 − 1  2  2 2 2 2 2     a b b b a a 8. Parametric representation: x = a secq , y = b tanq is a point on the hyperbola and this point is denoted by q. q is called a parameter of the hyperbola.

The equation of the tangent at q  is x secq − y tanq = 1.

a b ax by + = a2 + b2 . The equation of the normal at q  is secq tanq

9. The circle described on the transverse axis as diameter is called the auxiliary circle and its equation is x 2 + y 2 = a2 . 10. The equation of the director circle (the locus of the point of intersection of perpendicular tangents) is x 2 + y 2 = a2 − b2 .

Example 8.6.1 Find the equation of the hyperbola whose focus is (1, 2), directrix 2x + y = 1 and eccentricity 3.

Solution Let P (x1, y1) be any point on the hyperbola. Then

SP = e. S is (1, 2). PM

\ SP2 = (x1 - 1)2 + (y1 - 2)2 = x12 + y12 - 2x1 - 4y1 + 5 2x + y − 1 PM = ± 1 1 5 2  2 x + y − 1 SP2 = e2 PM 2, x12 + y12 − 2 x1 − 4 y1 + 5 = 3  1 1    5

Since

5 (x12 + y12 - 2x1 - 4y1 + 5) = 3 (4x12 + y12 + 1 + 4x1y1 - 4x1 - 2y1) (i.e.)  7x12 - 2 y12 + 12x1 y1 - 2x1 + 14y1 - 22 = 0 Hence, the equation of the hyperbola which is the locus of

(x1, y1) is 7x2 + 12xy - 2y2 - 2x + 14y - 22 = 0.

Example 8.6.2 Show that the equation of the hyperbola having focus (2, 0), eccentricity 2 and directrix x - y = 0 is x2 + y2 - 4xy + 4x - 4 = 0.

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8-6

Analytical Geometry

Solution S is (2, 0) : e = 2 and equation of the directrix is x - y = 0. Let P (x, y) be any SP = e. point on the hyperbola. Then, PM 2  x − y 2 2 2 2 2 SP = e PM ⇒ ( x − 2) + y = 4   2  x2 + 4 - 4x + y2 = 2 (x2 + y2 - 2xy) Hence, the equation of the hyperbola is x2 + y2 - 4xy + 4x - 4 = 0.

Example 8.6.3

3 Find the equation of the hyperbola whose focus is (2, 2), eccentricity and 2 directrix 3x - 4y = 1

Solution 3 S is (2, 2) : e = and directrix 3x - 4y = 1. Let P (x, y) be any point on the 2 hyperbola. SP = e or SP 2 = e 2 PM 2 PM



( x − 2)2 + ( y − 2)2 =

9  3 x − 4 y − 1   4 5

2

⇒ 100 (x2 - 4x + 4 + y2 - 4y +4) = 9 (9x2 + 16y2 + 1 - 24xy - 6x + 8y) Hence, the equation of the hyperbola is 19x2 + 216xy - 44y2 - 346x - 472y - 791 = 0.

Example 8.6.4

5 Find the equation of the hyperbola whose focus is (0, 0), eccentricity and 4 directrix x cosa + y sina = p

Solution

SP 2 = x 2 + y 2 : e = PM =

5 4

x cos a + y sin a − p cos 2 a + sin 2 a

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= x cos a + y sin a − p

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Hyperbola

8-7

For any point on the hyperbola, SP 2 = e 2 PM 2 ⇒ x 2 + y 2 =

25 ( x cos a + y sin a − p)2 16

Hence, the equation of the hyperbola is 16 (x2 + y2) - 25 (x cos a + y sin a - p)2 = 0.

Example 8.6.5 Find the equation of the hyperbola whose foci are (6, 4) and (-4, 4) and eccentricity 2.

Solution S is (6, 4) and S′ (-4, 4), and C is the midpoint of SS′ C

S′ (−4, 4)

(1, 4)

S (6, 4)

O

x

\ C is (1, 4)

CS = 6 - 1 = 5. But CS = ae = 2a. 5 ∴ 2a = 5 or a = 2 25 75 2 2 2 b = a (e − 1) = ×3= 4 4 Hence, the equation of the hyperbola is ( x − 1)2 ( y − 4)2 − =1 25 75 4 4 4( x − 1)2 4( y − 4)2 − =1 25 75

Example 8.6.6 Find the equation of the hyperbola whose center is (-3, 2), one end of the 5 transverse axis is (-3, 4) and eccentricity is . 2

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8-8

Analytical Geometry

Solution Centre is (-3, 4) A is (-3, 4) \ A′ is (-3, 6); a = 2  25  b 2 = a 2 (e 2 − 1) = 4  − 1 = 21  4 

Hence, the equation of the hyperbola is

( y − 2)2 ( x + 3)2 − = 1 (since the line parallel to y-axis is the transverse axis) 4 21 y2 + 4 − 4 y x2 + 9 + 6x − =1 4 21



-4x2 - 36 - 24x + 21y2 + 84 - 84y = 84



4x2 - 21y2 + 24x + 84y + 36 = 0

Example 8.6.7 Find the equation of the hyperbola whose centre is (1, 0), one focus is (6, 0), and length of transverse axis is 6.

Solution



2a = 6 \ a = 3  25  b 2 = a 2 (e 2 − 1) = 9  − 1 = 16  9  ae = 5

∴e =

Hence, the equation of the hyperbola is 32x - 128 = 0.

5 3

( x − 1)2 y 2 − = 1 (i.e.) 16x2 - 9y2 9 16

Example 8.6.8 Find the equation of the hyperbola whose centre is (3, 2), one focus is (5, 2) and one vertex is (4, 2).

Solution C is (3, 2), A is (4, 2) and S is (5, 2). Hence, CA = 1and the transverse axis is parallel to x-axis.

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Hyperbola



8-9

\a=1

Also ae = 2. Since a = 1 and e = 2, b2 = a2 (e2 - 1) = 1 (4 - 1) = 3. Hence, the equation of the hyperbola is

( x − 3)2 ( y − 2)2 − =1 1 3



3x2 - 18x + 27 - y2 + 4y - 4 = 3



3x2 - y2 - 18x + 4y + 20 = 0

Example 8.6.9 Find the equation of the hyperbola whose centre is (6, 2), one focus is (4, 2) and e = 2.

Solution Transverse axis is parallel to x-axis and CS = 2 units in magnitude.

ae = 2  \ a = 1



b2 = a2 (e2 - 1) = 1 (4 - 1) = 3

Hence, the equation of the hyperbola is

( x − 6)2 ( y − 2)2 − = 1. 1 3

Example 8.6.10 Find the centre, eccentricity and foci of hyperbola 9x2 - 16y2 = 144.

Solution Dividing by 144, we get

x2 y2 − =1 16 9 Hence, the centre of the hyperbola is (0, 0)



\ a2 = 16, b2 = 9  \a = 4, b = 3 b 2 = a 2 (e 2 − 1) ⇒ 9 = 16(e 2 − 1) ⇒ e 2 − 1 =

9 16

5 4 Hence, the foci are (5, 0) and (-5, 0).

∴ e=

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8-10

Analytical Geometry

Example 8.6.11 Find the centre, foci and eccentricity of 12x2 - 4y2 - 24x + 32y - 127 = 0

Solution

(12x2 - 24x) - (4y2 - 32y) - 127 = 0



12 (x2 - 2x) - 4 (y2 - 8y) - 127 = 0



12 (x- 1)2 - 12 - 4 (y - 4)2 + 64 - 127 = 0 12 (x - 1)2 - 4(y - 4)2 = 75



( x − 1)2 ( y − 4)2 =1 −  75   75      12 4



Hence, centre is (1, 4). 75 2 75 ,b = 12 4 b 2 = a 2 (e 2 − 1) 75 75 2 = (e − 1) 4 12 12 ⇒ e2 − 1 = =3 4 ⇒ e 2 = 4 or e = 2



a2 =

CS = ae =

5 3 2 3

×2 =

5 × 2 =5 2

Hence, the foci are (6, 4) and (- 4, 4).

Example 8.6.12 Find the centre and eccentricity of the hyperbola 9x2 - 4y2 + 18x + 16y - 43 = 0.

Solution

9 (x2 + 2x) - 4 (y2 - 4y) - 43 = 0 9 (x + 1)2 - 9 - 4(y - 2)2 + 19 - 43 = 0

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Hyperbola

8-11

9 (x + 1)2 - 4 (y - 2)2 = 36 2 Hence, centre is (-1, 2), a = 4 and b2 = 9. ( x + 1)2 ( y − 2)2 − =1 4 9 13 b2 = a2 (e2 - 1) gives e = . 2

Example 8.6.13 If from the centre C of the hyperbola x2 - y2 = a2, CM is drawn perpendicular to the tangent at any point of the curve meeting the tangent at M and the curve at N, show that CM ⋅ CN = a2.

Solution The equation of the tangent at P (x1, y1) in x2 - y2 = a2 is xx1 - yy1 = a2. y P(x1, y 1) x

C M

N

The equation of the line CN is xy1 + yx1 = 0 Then CM, which is perpendicular from C on the tangent, is given by CM =



a2 x12 + y12

Solving x2 - y2 = a2 and xy1 + yx1 = 0 we get the coordinates of N x2 −

x 2 y12 = a 2 ⇒ x 2 ( x12 − y12 ) = a 2 , x12 x12 ax1 ∴x = ± = ± x1 ∴ y = ± y1 x12 − y12 ∴ N is ( ± x1, ± y1 )

∴ CM × CN =

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a2 x +y 2 1

2 1

⋅ x12 + y12 = a 2

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8-12

Analytical Geometry

Example 8.6.14 Tangents to the hyperbola make angles q1, q2 with the transverse axis. Find the equation of the locus of point of intersection such that tan (q1 + q2) is a constant.

Solution

x2 y2 − = 1. Then, the equation of the a2 b2 tangent to the hyperbola is y = mx + a 2 m2 − b 2 . Let the equation of the hyperbola be

If this tangent passes through (x1, y1), then y1 = mx1 + a 2 m2 − b 2 . (y1 - mx1)2 = a2 m2 - b2 m2 (x12 -a2) - 2mx1 y1+ y12 + b2 = 0



If m1 and m2 are the slopes of the two tangents, then m1 + m2 =



−2 x1 y1 x12 − a 2

, m1m2 =

y12 + b 2 . x12 − a 2

It is given that tan (q1 + q2) = k. tan q1 + tan q 2 = k 1 − tan q1 tan q 2



(i.e.)  m1 + m2 = k (1 - m1 m2) (i.e.) 

 −2 x1 y1 y12 + b 2  = k − 1 ⇒ −2 x1 y1 = k ( x12 − y12 − a 2 − b 2 )  x12 − a 2 x12 − a 2 

Hence, the locus of (x1, y1) is k (x2 + y2 - a2 - b2) 2xy = 0.

Example 8.6.15 Prove that two tangents that can be drawn from any point on the hyperbola x2 y2 x2 - y2 = a2 - b2 to the ellipse 2 + 2 = 1 which make complementary angles a b with the axes.

Solution The tangent drawn from any point to the ellipse

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x2 y2 + = 1 is a2 b2

y = mx + a 2 m2 − b 2

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Hyperbola

8-13

Since this passes through (x1, y1)

y1 = mx1 + a 2 m2 − b 2 ⇒ (y1 - mx1)2 = a2 m2 - b2 2

(i.e.)  m2 ( x1 - a2) - 2mx1y12 + y12 - b2 = 0. If m1 and m2 are the slopes of the tangents, then y 2 − b2 m1m2 = 12 x1 − a 2 Since (x1, y1) lies on x2 - y2 = a2 - b2, we have x12 - y12 = a2 - b2. 2 \ x1 - a2 = y12 - b2 y12 − b 2 =1 x12 − a 2 \ m1m2 = 1 Hence, the two tangents make complementary angles with the axes.



Example 8.6.16

x2 y2 − = 1 are at a constant distance from the cena2 b2 tre. Find the locus of their poles. Chords of the hyperbola

Solution

x2 y2 Let (x1, y1) be the pole with respect to hyperbola 2 − 2 = 1. The polar of a b xx yy (x1, y1) is 21 − 21 = 1. The perpendicular distance from the centre on the a b x2 y2 1 1 polar is = k ( a constant) (i.e.) 14 + 14 = . k a b x12 y12 + 2 2 a b x2 y2 1 Hence, the locus of (x1, y1) is 4 + 4 = . k a b

Example 8.6.17

x2 y2 Find the equation of common tangents to the hyperbolas 2 − 2 = 1 and a b y2 x2 − 2 = 1. 2 a b

Solution

x2 y2 x2 y2 − 2 = 1 and − 2 = 1. 2 2 a b −b −a The conditions for y = mx + c to be a tangent to the hyperbolas are c2 = a2 m2 - b2 The two given hyperbolas are

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8-14

Analytical Geometry

and

c2 = - b2 m2 + a2 \ a2 m2 - b2 = - b2 m2 + a2



m2 (a2 + b2) = a2 + b2



(i.e.) (m2 - 1) (a2 + b2) = 0



\m=±1

Hence, there are two common tangents whose equations are y = ± x ± a 2 − b 2 .(i.e.) y = ± ( x + = ± x ± a 2 − b 2 (i.e.) y = ± ( x + a 2 − b 2 ) .

Example 8.6.18 Show that the locus of midpoints of normal chords of the hyperbola x2 - y2 = a2 is (y2 - x2)3 = 4a2 x2 y2.

Solution The equation of the hyperbola is x2 - y2 = a2. Let (x1, y1) be the midpoint of a normal chord of the hyperbola. The equaax ay tion of the normal is + = 2a 2 and the equation of the chord in secq tan q terms of the middle point is xx1 - yy1 = x12 - y12. Both these equations represent the same line. Hence, identifying them, we get a /secq a /tan q 2a 2 = = 2 x1 − y1 x1 − y12





∴ secq =

x12 − y12 a ( x12 − y12 ) x12 − y12 = and tan q = − 2ax1 2ay1 2a 2 x1

Squaring and subtracting, we get



( x12 − y12 ) ( x12 − y12 )2 − =1 4 a 2 x12 4 a 2 y12 ( x12 − y12 )2  1 1 − 2 =1 2 2  4a  x1 y1 

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Hyperbola

8-15

⇒ (y12 - x12 )3 = 4a2 x12 y12 The focus of (x1, y1) is (y2 - x2)3 = 4a2x2 y2.

Example 8.6.19

x2 y2 − =1 a2 b2 h k passing through a fixed point (h, k) is a hyperbola whose centre is  ,  .  2 2

Prove that the locus of middle points of chords of the hyperbola

Solution

x2 y2 − = 1. a2 b2 The equation of the chord of the hyperbola in terms of its middle point is xx1 yy1 x12 y12 − 2 = 2− 2 a2 b a b Since this chord passes through the fixed point ( h, k ),

The equation of the hyperbola is

xh yk x12 y12 − = − a2 b2 a2 b2

2

(i.e.)

2

1  h2 k 2   x1 h   y1 k   a − 2  −  b − 2  = 4  a 2 − b 2    2

2

1  h2 k 2   x h  y k  The locus of (x1, y1) is  −  −  −  =  2 − 2  , which is a  a 2  b 2  4a b  h k ­hyperbola whose centre is  ,  .  2 2

Example 8.6.20 Show that the locus of the foot of the perpendicular from the centre upon any  a2 b2  ( a2 + b2 )2 x2 y2 normal to the hyperbola 2 − 2 = 1 is ( x 2 + y 2 )2  2 − 2  = . y a b y  x

Solution Let P (a sec q, b tan q ) be a point on the hyperbola. Let m (x1, y1) be the foot of the perpendicular from the centre with normal at P. The equation of the ax by normal at P is + = a2 + b2. secq tan q

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8-16

Analytical Geometry

M P

The equation of the perpendicular from C (0, 0) on this normal is b a x− y = 0 tan q secq These two lines intersect at (x1, y1)

ax1 by1 + = a 2 + b 2 and secq tan q bx1 ay1 − = 0 tan q secq Solving these two equation for x1 and y1 we get ∴





x1 −( a + b ) a sec q 2

2

x1 sec q a( a 2 + b2 )

=

=

x1 =

y1 −b( a + b ) tan q 2

2

y1 tan q b( a 2 + b 2 )

=

1

= −

2

a b2 − 2 sec q tan 2 q

sec2 q tan 2 q a 2 tan 2 q + b2 sec2 q

b( a 2 + b 2 ) sec2 q tan q a( a2 + b2 ) sec q tan 2 q , y1 = 2 2 2 2 2 a tan q + b sec q a tan 2 q + b 2 sec2 q

a 2 ( a 2 tan 2 q + b 2 sec2 q )2 b 2 ( a 2 tan 2 q + b 2 sec2 q )2 = , = x12 ( a 2 + b 2 )2 sec2 q tan 4 q y12 ( a 2 + b 2 )2 sec 4 q tan 2 q



 a 2 b 2  ( a 2 tan 2 q + b 2 sec2 q )2  sec2 q − tan 2 q   sec 4 q tan 4 q   x 2 − y 2  = ( a2 + b2 )2 1 1 x12 + y12 =

 a 2 sec2 q tan 4 q +  ( a2 + b2 )2 ( a 2 tan 2 q + b 2 sec2 q )2  b 2 sec 4 q tan 2 q 

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Hyperbola



=

( a 2 + b 2 )2 sec2 q tan 2 q 2 a tan 2 q + b 2 sec2 q ( a 2 tan 2 q + b 2 sec2 q )2

)



( x12 + y12 )2 =

( a 2 + b 2 ) 4 sec 4 q tan 4 q 2 a tan 2 q + b 2 sec2 q 2 2 2 2 4 ( a tan q + b sec q )

)

(

(

8-17

2

 a 2 b 2  ( a 2 + b 2 )2 sec 4 q tan 4 q  1  ( x12 + y12 )2  2 − 2  = 2    x1 y1  ( a tan 2 q + b 2 sec2 q )2  sec 4 q tan 4 q  ( a 2 tan 2 q + b 2 sec2 q )2

= (a2 + b2)2

 a2 b2  Hence, the locus of (x1, y1) is ( x 2 + y 2 )2  2 − 2  = ( a 2 + b 2 )2 . y  x

Example 8.6.21

x2 y2 − = 1. Prove that a2 b2 their middle points lie on the curve (x2 + y 2)2 = a2 x2 - b2 y2.

Chords of the curve x2 + y2 = a2 touch the hyperbola

Solution Let (x1, y1) be the midpoint of the chord of the circle. Its equation is

xx1 + yy1 = x12 + y12 (i.e.)



yy1 = − xx1 + x12 + y12

(i.e.)

This is a tangent to the hyperbola

y=

− xx1 x12 + y12 + y1 y1

x2 y2 − = 1. a2 b2

Hence, the condition is 2



2  x12 + y12  2 x1 a = − b2  y  y12 1

(i.e.)  (x12 + y12)2 = (a2 x12 - b2 y12) Hence, the locus of (x1, y1) is (x2 + y2)2 = (a2 x2 - b2y2).

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8-18

Analytical Geometry

Example 8.6.22 Show that the locus of midpoints of normal chords of the hyperbola x2 - y2 = a2 is (y2 - x2)2 = 4a2 xy.

Solution Let (x1, y1) be the midpoint of the normal chord of the hyperbola x2 - y2 = a2. Then, the equation of the chord is xx1 - yy1 = x12 - y12



The equation of the normal at ‘q ’ is ax ay + = 2a 2 secq tan q



These two equations represent the same line. Identifying, we get x1 secq − y1 tan q x12 − y12 = = a a 2a 2 ∴ secq =



x12 − y12 x 2 − y12 , , tan q = 1 2ax1 2ay1

sec2 q − tan 2 q = 1 ⇒

(i.e.)  1 =

( x12 − y12 )2 4a2

( x12 − y12 )2 ( x12 − y12 )2 − =1 4 a 2 x12 4a 2 y12

1 1 2 2 3 2 2 2  2 − 2  ⇒ ( y1 − x1 ) = 4a x1 y1 x y  1 1 

The locus of (x1, y1) is (y2 - x2)3 = 4a2 x2 y2.

Example 8.6.23

x2 y2 − = 1 meets the axes at Q and R and lines a2 b2 QL and RL are drawn at right angles to the axes and meet at L. Prove that the locus of the point L is the hyperbola (a2 x2 - b2 y2) = (a2 + b2)2. Prove further that the locus of the middle point of QR is 4 (a2 x2 - b2 y2) = (a2 + b2)2. A normal to the hyperbola

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Hyperbola

8-19

Solution y

R P

L(x1, y1)

Q

x

x2 y2 Let P (h, k) be the point on the hyperbola 2 − 2 = 1. The equation of the a b a2 x b2 y 2 2 normal at (h, k) is + = a + b . When this line meets the x-axis y = 0 h k  ( a2 + b2 )h  Therefore, the coordinates of Q are  , 0 . The coordinates of R a2    ( a2 + b2 )k  h . Let (x1, y1) be the coordinates of L. Then, x1 = ( a 2 + b 2 ) 2 are  0, 2  b a   k 2 2 and y1 = ( a + b ) 2 . b h k ∴ ax1 = ( a 2 + b 2 ) and by1 = ( a 2 + b 2 ) a b 2 2 h k  ∴ a 2 x12 − b 2 y12 = ( a 2 + b 2 )2  2 − 2  = ( a 2 + b 2 )2 since (h, k) lies on the b  a ­hyperbola. The locus of (x1, y1) is a2 x2 - b2 y2 = (a2 + b2)2. ( a2 + b2 )k ( a2 + b2 )h and b = . Let (a , b  ) be the midpoint of QR. Then, a = 2 2a 2b 2 2 2 x y  ∴ 4( a 2a 2 − b 2 b 2 ) = ( a 2 + b 2 )2  2 − 2  = ( a 2 + b 2 )2 since (h, k) lies on b  a the hyperbola. The locus of (a , b  ) is 4 (a2 x2 - b2 y2) = (a2 + b2)2.

Example 8.6.24 The chords of the hyperbola x2 - y2 = a2 touch the parabola y2 = 4ax. Prove that the locus of their midpoint is the curve y2 (a - y) = x3.

Solution

x2 y2 Let (x1, y1) be the midpoint of the chord of the hyperbola 2 − 2 = 1. Its a a xx yy x2 y2 equation is 21 − 21 = 12 − 12 . a a a a

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8-20

Analytical Geometry

xx1 - yy1 = x12 - y12 ⇒ yy1 = - x1 x - (x12 - y12)



⇒y=



( x 2 − y12 ) x1 x− 1 y1 y1

The line is a tangent to the parabola y2 = 4ax. The condition is −( x12 − y12 ) ay1 = y1 x1



(i.e.)  -x1 (x12 - y12) = ay12



The locus of (x1, y1) is x (x2 - y2) = ay2 (i.e.) y2 (a - y) = x8.

Example 8.6.25

x2 y2 − = 1 meets the transverse axis at a2 b2 Q and the tangent at the vertex at R. Show that the locus of the midpoint QR is x (4y2 + b2) = ab2.

A variable tangent to the hyperbola

Solution The equation of the tangent at ‘q  ’ is

x y secq − tan q = 1. a b

y R

A′

C

Q

A (a, 0)

x

When this line meets the transverse axis, y = 0 and x = a cosq. Here Q is (a cos q, 0). When it meets the line x = a,

secq − 1 =

y b(1 − cos q ) tan q ∴ y = ∴ R is b sin q

 b(1 − cos q )   a,  sin q

Let (h, k) be the midpoint of QR. Then,

M08_ANALYTICALGEOMETRY_ISBN_CH08.indd 20

h=

a cosq + a 2

4/22/2013 8:36:39 PM

Hyperbola

∴ 2h = a(1 + cos q ) or cos q =



k=

8-21

2h − a a

b(1 − cos q ) 2 sin q

b(1 − cos q ) sin q 4 hk 2h − a , cos q = 4 hk = ab sin q ⇒ sin q = a ab 2 2 2 (2h − a) 16h k Squaring and adding, 1 = + 2 2 a2 ab 2 2 2 2 a b = b (2h − a) + 16h2 k 2 ⇒ 4 h2 b 2 a2 − 4b2 ha + 16h2 k 2 = 0 ∴ hb2 − ab2 + 4 hk 2 = 0 ∴ 2h = a(1 + cos q ), 2k =



Hence, the locus of ( h, k ) is b 2 x + 4 xy 2 = ab 2 or x(b 2 + 4 y 2 ) = ab 2.

Example 8.6.26 x2 y2 − =1 a2 b2 2  x2 y2   1 1  x2 y2 that subtends a right angle at the centre is  2 − 2   2 − 2  = 4 + 4 . b  a b  a b a

Show that the locus of the midpoints of the chords of the hyperbola

Solution Let P ( x1 , y1 ) be the midpoint of a chord of the hyperbola

x2 y2 − = 1. a2 b2

P

y

x R

P(x1, y1)

Q

Then, the equation of the chord is T = S1 (i.e.) 

xx1 yy1 x12 y12 − 2 = 2− 2 a2 b a b

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8-22

Analytical Geometry

The chord subtends a right angle at the centre of the hyperbola. Hence, the combined equation of the lines CP and CQ is  x2 y2  − = a2 b2  



xx1 yy1  − 2  a2 b  2 x1 y12  − a2 b2 

2



 x2 y2   x2 y2   xx yy  (i.e.)   2 − 2   12 − 12  =  21 − 21  a b a b  b  a

2

2

Since ∠QCR = 90°, coefficient of x2 + coefficient of y2 = 0.

 1  x2 y2  2 x2  1  x2 y2  2 y2  (i.e.)    2  12 − 12  − 14  − 2  12 − 12  + 14  = 0 b  a  b a b  b   a  a   



 x2 y2   1 1  x2 y2 (i.e.)   12 − 12   2 − 2  = 14 − 14 b  a b  a b a

2

2

 x2 y2   1 1  x2 y2 The locus of (x1, y1) is  2 − 2   2 − 2  = 4 − 4 . b  a b  a b a

Example 8.6.27 From points on the circle x2- y2 = a2 tangents are drawn to the hyperbola x2 - y2 = a2. Prove that the locus of the middle points of the chords of contact is the curve (x2 - y2) = a2 (x2 + y2).

Solution Let P (x1, y1) be a point on the circle x2 + y2 = a2. y

Q

R C

x

P



M08_ANALYTICALGEOMETRY_ISBN_CH08.indd 22

\ x12 + y12 = a2

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Hyperbola

8-23

Let (h, k) be the midpoint of the chord of contact QR of the tangents from P to the hyperbola x2 - y2 = a2. Then the equation of chord of contact to the hyperbola is xx1 - yy1 = a2



The equation of the chord in terms of the middle point (h, k) is xh - yk = h2 - k



These two equations represent the same line. Identifying them, we get x1 y1 a2 = = 2 h k h − k2



\ x1 = x12 + y12 =

a2 h h2 − k 2

a 2 ( h2 + k 2 ) ( h2 − k 2 ) 2

and

(i.e.) a 2 =

y1 =

a2 k h2 − k 2

a 4 ( h2 + k 2 ) (since x12 + y12 = a 2 ) ( h2 − k 2 ) 2

Hence, the locus of (h, k) is (x2 - y2)2 = a2 (x2 + y2).

Example 8.6.28 x2 y2 − = 1 meet on a2 b2 the conjugate axis at Q and R, show that the circle described with QR as the diameter passes through the foci of the hyperbola.

If the tangent and normal at any point of the hyperbola

Solution

x2 y2 The equation of the tangent and normal at ( x1 , y1 ) on the hyperbola 2 − 2 = 1 a b are xx1 yy1 − 2 =1 (8.3) a2 b

a2 x b2 y + = a2 + b2 x1 y1

(8.4)

These two lines meet the conjugate axis at Q and R. Therefore substitute  −b2  . The x = 0 in equations (8.3) and (8.4). The coordinates of Q are  0, y1   y   coordinates of R are  0,( a 2 + b 2 ) 21  .  b 

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8-24

Analytical Geometry

The equation of the circle with QR as diameter is

 ( a 2 + b 2 )2 y1  b2   x2 +  y +   y −  = 0 y1   b2 

Substituting x = ± ae and y = 0 a2 e2 −



b 2 ( a 2 + b 2 ) y1 =0 y1 b2

(i.e.) a 2 e 2 − ( a 2 + b 2 ) = 0 (i.e.) a 2 e 2 − a 2 e 2 = 0 which is true. Hence, the circle with QR as diameter passes through the foci.

Exercises 1.  2. 3.  4.  5.  6.

7.  8.

Find the equation of the hyperbola whose focus is (1, 2), directrix 2x + y = 1 and eccentricity 3 . Ans.: 7x2 + 12xy - 2y2 - 2x + 14y - 22 = 0 Show that the equation of the hyperbola having focus (2, 0), eccentricity 2 and directrix x - y = 0 is x2 + y2 - 4xy + 4 = 0. 3 Find the equation of the hyperbola whose focus is (2, 2), eccentricity and 2 directrix 3x - 4y = 1. Ans.: 19x2 + 44y2 - 216xy - 346x + 472y - 791 = 0 5 Find the equation of the hyperbola whose focus is (0, 0), eccentricity and 4 directrix x cos a + y sin a = p. Ans.: 16(x2 + y2) - 25(xcos a  + ysin a - p)2 = 0 Find the equation of the hyperbola whose centre is (-3, 2) and one end of the 5 transverse axis is (-3, 4) and eccentricity is . 2 Ans.: 4x2 - 21y2 + 24x + 84y + 36 = 0 Find the equation of the hyperbola whose foci are (6, 4) and (-4, 4) and 4( x − 1)2 4( y − 4)2 eccentricity 2. Ans.: − =1 25 75 Find the equation of the hyperbola whose centre is (1, 0), one focus is (6, 0) and length of transverse axis is 6. Ans.: 16x2 - 9y2 - 32x - 128 = 0 Find the equation of the hyperbola whose centre is (3, 2), one focus is (5, 2) Ans.: 3x2 - y2 - 18x - 4y + 20 = 0 and one vertex is (4, 2).

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8-25

Hyperbola

9.

10.  11.

Find the equation of the hyperbola whose centre is (6, 2), one focus is (4, 2) ( x − 6)2 ( y − 2)2 and eccentricity 2. Ans.: − =1 1 3 Find the centre, eccentricity and foci of hyperbola 9x2 - 16y2 = 144. 5 Ans.: (0, 0), ,(5, 0) and ( −5, 0) 4 Find the centre, foci and eccentricity of 12x2 - 4y2 - 24x + 32y - 127 = 0. Ans.: (1, 4), (6, 4) and (-4, 4)

 12.

Find the centre, foci and eccentricity of the hyperbola 9x2 - 4y2 - 18x + 16y - 43 = 0. Ans.: ( −1, 2),( 13 , 0) and ( − 13 ,0),

 13. 14. 15.

13 2

If S and S′ are the foci of a hyperbola and p is any point on the hyperbola, show that S′P - SP = 2a. x2 y2 2b 2 Find the latus of the hyperbola 2 − 2 = 1 . Ans.: a b a Find the equation of the hyperbola referred to its axis as the axis of coordinate if length of transverse axis is 5 and conjugate axis is 4. Ans.:



x2 y2 − =1 25 16 8 3

16.

Find the latus rectum of the hyperbola 4x - 9y2 = 36.

17.

Find the centre, eccentricity and foci of the hyperbola x 2 − 2 y 2 − 2 x + 8 y − 1 = 0. Ans.: (1, 2), 3 ,(1, 5),(1, −1), y = 5, x = −1

 18.

Ans.:

Find the centre, eccentricity, foci and directrix of the hyperbola 16 x 2 − 9 y 2 + 32 x + 36 y − 164 = 0. 5 Ans.: ( −1, 2), ,( −6, 2),( 4,1), 5 x = 4, 5 x = −14 3



x2 y2 − = 1 passes through the intersection of the lines a2 b2 32 2 . Find a 7 x + 13 y − 87 = 0 and 5 x − 8 y + 7 = 0 and its latus rectum is 5 5 2 and b. Ans.: ,4 2

19.

The hyperbola

20.

Tangents are drawn to the hyperbola 3 x 2 − 2 y 2 = 6 from the point P and make q1 ,q 2 with x-axis. If the tanq1 tanq 2 is a constant, prove that locus of P is 2 x 2 − y 2 = 7.

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21.

Analytical Geometry

Find the equation of tangents to the hyperbola 3 x 2 − 4 y 2 = 15 which are parallel to y = 2 x + k . Find the coordinates of the point of contact. Ans.: y = 2 x ±

 22.

23.

24.

65  20 5   −20 −5  , , , , 2  65 2 65   65 2 65 

Tangents are drawn to the hyperbola x 2 − y 2 = c 2 are inclined at an angle of 45°, show that the locus of their intersection is ( x 2 + y 2 )2 + 4 a 2 ( x 2 − y 2 ) = 4 a 4 . x2 y2 Prove that the polar of any point on the ellipse 2 + 2 = 1 with respect to the a b x2 y2 hyperbola 2 − 2 = 1 will touch the ellipse at the other end of the ordinate a b through the point. If the polar of points ( x1 , y1 ) and (x2 , y2 ) with respect to hyperbola are at right angles then show that b 4 x1 x2 + a 4 y1 y2 = 0.

25.

Find the locus of poles of normal chords of the hyperbola

26.

Chords of the hyperbola

x2 y2 − =1. a2 b2

x2 y2 − = 1 subtend a right angle at one of the a2 b2 vertices. Show that the locus of poles of all such chords is the straight line x( a 2 + b 2 ) = a( a 2 − b 2 ).

27.

If chords of the hyperbola are at a constant distance k from the centre, find x2 y2 1 the locus of their poles. Ans.: 4 + 4 = 2 b k a

28.

Obtain the locus of the point of intersection of tangents to the hyperbola x2 y2 − = 1 which includes an angle b . a2 b2 Ans.: 4( a 2 y 2 − b 2 x 2 + a 2b 2 ) = ( x 2 + y 2 − a 2 + b 2 ) tan 2 b

 29.

x2 y2 − = 1 is a tangent to the circle a2 b2 2 2 2 x + y = c then prove that the locus of its middle point is

If a variable chord of the hyperbola

2

2  x2 y2  y2  2 x . − = + c 2 2 4  a  a b  b 4 

30.

Show that the condition for the line x cos a + y sin a = b touches the hyperbola x2 y2 − = 1 is a 2 cos 2 a − b 2 sin 2 a = p 2. a2 b2

31.

Prove that the tangent at any point bisects the angle between focal distances of the point.

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Hyperbola

32. 33.

34.

35.

x2 y2 Prove that the midpoints of the chords of the hyperbola 2 − 2 = 1 parallel a b to the diameter y = mx be on the diameter a 2 my = b 2 x. If the polar of the point A with respect to a hyperbola passes through another point B, then show that the polar B passes through A. x2 y2 If the polars of ( x1 , y1 ) and ( x2 , y2 ) with respect to the hyperbola 2 − 2 = 1 a b x1 x2 a4 are at right angles, then prove that + 4 = 0. y1 − y2 b Prove that the polar of any point on x2 y2 x2 y2 − 2 = 1 touches 2 + 2 = 1 . 2 a b a b

36.

37.

8-27

x2 y2 + = 1 with respect to the hyperbola a2 b2

Obtain the equation of the chord joining the points q and f on the hyperbola in x q − f  y q + f   q + f  . If q - f is a constant the form cos  − sin = cos   2  b  2   2  a x 2 cos 2 a y 2 − 2 =1. and equal to 2a, show that PQ touches the hyperbola a2 b If a circle with centre (3a , 3b ) and of variable radius cuts the hyperbola x 2 − y 2 = 9a 2 at the points P , Q , R and S then prove that the locus of the centroid of the triangle PQR is ( x − 2a )2 − ( y − 2 b )2 = a 2 .

38.

If the normal at P meets the transverse axis in r and the conjugate axis in g and CF be perpendicular to the normal from the centre then prove that PF ⋅ Pr = CB 2 and PF ⋅ Pg = CF 2 .

39.

Show that the locus of the points of intersection of tangents at the extremities 6 6 x2 y2 of normal chords of the hyperbola 2 − 2 = 1 is a − b = ( a 2 + b 2 )2 . 2 a b x y2 Find the equation and length of the common tangents to hyperbolas x2 y2 y2 x2 − = 1 and − =1. a2 b2 a2 b2   1 Ans.:  ; x − y = a2 − b2  2 2  a −b 

40.

 41.

42.

Tangents are drawn from any point on hyperbola x 2 − y 2 = a 2 + b 2 to the x2 y2 hyperbola 2 − 2 = 1 . Prove that they meet the axes in conjugate points. a b Prove that the part of the tangent at any point of a hyperbola intercepted between the point of contact and the transverse axis is a harmonic mean between the lengths of the perpendiculars drawn from the foci on the normal at the same point.

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Analytical Geometry

x2 y2 − = 1 is a a2 b2 a b ke − 1 focal chord then prove that tan tan + = 0 where k ≠ 1. 2 2 ke + 1

43.

If the chord joining the points a and b on the hyperbola

44.

Let the tangent and normal at a point P on the hyperbola meet the transverse axis in T and G respectively, prove that CT ⋅ CG = a 2 + b 2 .

45.

If the tangent at the point ( h, k ) to the hyperbola cuts the auxiliary circle in 1 1 2 points whose ordinates are y1 and y2 then show that + = . y1 y2 k

46.

If a line is drawn parallel to the conjugate axis of a hyperbola to meet it and the conjugate hyperbola in the points P and Q then show that the tangents at 4 2 2 2 P and Q meet on the curve y − x = 4b x . b2 a2 x2 yn

47.

48.

If an ellipse and a hyperbola have the same principal axes then show that the polar of any point on either curve with respect to the other touches the first curve. x2 y2 If the tangent at any point P on the hyperbola 2 − 2 = 1 whose centre is C, a b meets the transverse and conjugate axes in T1 and T2 , then prove that (i) CN ⋅ CT1 = a 2 and (ii) CM ⋅ CT2 = −b 2 where PM and PN are perpendiculars in the transverse and conjugate axes, respectively.

49.

If P is the length of the perpendicular from C, the centre of the hyperbola x2 y2 − = 1 on the tangent at a point P on it and CP = r , prove that a2 b2 a 2b 2 = r 2 + b2 − a2 . p2

8.7  ASYMPTOTES Definition 8.7.1 An asymptote of a hyperbola is a straight line that touches the hyperbola at infinity but does not lie altogether at infinity. 8.7.1  Equations of Asymptotes of the Hyperbola

x2 y2 − = 1. Let y = mx + c be an a2 b2 asymptote of the hyperbola. Solving these two equations, we get their points of intersection. The x coordinates of the points of intersection are given by Let the equation of the hyperbola be

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(i.e.)

8-29

x 2 ( mx + c)2 − =1 a2 b2 b 2 x 2 − a 2 ( mx + c)2 = a 2 b 2

(i.e.)  x 2 (b 2 − a 2 m2 ) − 2mca 2 x − a 2 (b 2 + c 2 ) = 0

If y = mx + c is an asymptote, then the roots of the above equation are infinite. The conditions for these are the coefficient of x 2 = 0 and the coefficient of x = 0, b 2 − a 2 m2 = 0 and mca 2 = 0. b (i.e.)  m = ± and c = 0 a b The equations of the asymptotes are y = ± x. a x y x y (i.e.) − = 0 and + = 0 a b a b The combined equation of the asymptotes is

Note 8.7.1.1:

x2 y2 − = 0. a2 b2

x2 y2 1. The asymptotes of the conjugate hyperbola 2 − 2 = −1 are also given by a b x2 y2 − = 0 . Therefore, the hyperbola and the conjugate hyperbola have a2 b2 the same asymptotes. x2 y2 2. The equation of the hyperbola is H : 2 − 2 − 1 = 0. a b 2 x y2 The equation of the asymptotes is A : 2 − 2 = 0. a b x2 y2 The equation of the conjugate hyperbola is C : 2 − 2 + 1 = 0. a b 3. The equation of the asymptotes differs from that of the hyperbola by a constant and the equation of the conjugate hyperbola differs from that of the asymptotes by the same constant term. This result holds good even when the equations of the hyperbola and its asymptotes are in the most general form. 4. The asymptotes pass through the centre (0, 0) of the hyperbola. b −b . 5. The slopes of the asymptotes are and a a Hence, they are equally inclined to the coordinate axes, which are the transverse and conjugate axes.

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Analytical Geometry

8.7.2  Angle between the Asymptotes y

q

C

x

q

Let 2q  be the angle between the asymptotes. Then, m = tanq =



b a

sec2q = 1 + tan 2q = 1 +



b2 a2 + b2 = = e2 a2 a2

∴ secq = e Hence, the angle between the asymptotes is 2 sec −1 (e).

Example 8.7.1 Find the equation of the asymptotes of the hyperbola 3 x 2 − 5 xy − 2 y 2 + 17 x + y + 14 = 0.

Solution The combined equation of the asymptotes should differ from that of the hyperbola only by a constant term. \ The combined equation of the asymptotes is 3 x 2 − 5 xy − 2 y 2 + 17 x + y + k = 0

3 x 2 − 5 xy − 2 y 2 = 3 x 2 − 6 xy + xy − 2 y 2 = 3 x( x − 2 y ) + y( x − 2 y ) = (3 x + y )( x − 2 y )

Hence, the asymptotes are 3 x + y + l = 0 and x − 2 y + m = 0.

(3 x + y + l )( x − 2 y + m) = 3 x 2 − 5 yx − 2 y 2 + 17 x + y + k

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Hyperbola

Equating the coefficients of the terms x and y and the constant terms, we get l + 3m = 17



−2l + m = 1



Solving these two equations, we get l = 2 and m = 5. lm = k



\ k = 10. The combined equation of the asymptotes is (3 x + y + 2) ( x − 2 y + 5) = 0.

Example 8.7.2 Find the equation of the asymptotes of the hyperbola xy = xh + yk .

Solution The combined equation of the asymptotes is xy = xh + yk + n or xy − xh − yk − n = 0. The asymptotes are x + l = 0 and y + m = 0.

( x + l )( y + m) = xy − xh − yk − n

Equating the coefficients of the terms x and y and the constant terms, we get

m = − h, l = − k ∴ n = − hk

and lm = − n

Hence, the equation of the asymptotes is ( x − h)( y − k ) = 0.

Example 8.7.3 Find the equation to the hyperbola that passes through (2, 3) and has for its asymptotes the lines 4 x + 3 y − 7 = 0 and x − 2 y = 1.

Solution The combined equation of the asymptotes is ( 4 x + 3 y − 7)( x − 2 y − 1) = 0. Hence, the equation of the hyperbola is ( 4 x + 3 y − 7)( x − 2 y − 1) + k = 0. This pass through (2, 3).

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Analytical Geometry

(8 + 9 − 7)(2 − 6 − 1) + k = 0 ∴ k = 50



Hence, the equation of the hyperbola is ( 4 x + 3 y − 7)( x − 2 y − 1) + 50 = 0



(i.e.)

4 x 2 − 5 xy − 6 y 2 − 11x + 11 y + 57 = 0

Example 8.7.4 Find the equation of the hyperbola that has 3 x − 4 y + 7 = 0 and 4 x + 3 y + 1 = 0 as asymptotes and passes through the origin.

Solution The combined equation of the asymptotes is (3 x − 4 y + 7)( 4 x + 3 y + 1) = 0 Hence, the equation of the hyperbola is (3 x − 4 y + 7)( 4 x + 3 y + 1) + k = 0. This passes through the origin (0, 0). ∴ 7 + k = 0 or k = −7 Hence, the equation of the hyperbola is (3 x − 4 y + 7)( 4 x + 3 y + 1) − 7 = 0 12 x 2 − 12 y 2 − 7 xy + 31x + 17 y = 0

Example 8.7.5 Find the equations of the asymptotes and the conjugate hyperbola given that the hyperbola has eccentricity 2, focus at the origin and the directrix along x + y + 1 = 0.

Solution From the focus directrix property, the equation of the hyperbola is 2

 x + y + 1 x2 + y2 = 2    2  (i.e.)  2 xy + 2 x + 2 y + 1 = 0 The combined equation of the asymptotes is 2 xy + 2 x + 2 y + k = 0, where k is a constant. Let the asymptotes be 2 x + l = 0 and y + m = 0. Then,

2 xy + 2 x + 2 y + k = (2 x + l )( y + m) Equating like terms, we get 2m = 2. ∴ m = 1. Similarly, l = 2. As lm = k, we get k = 2.

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Therefore, the asymphtes of the combined equation of the asymptotes is 2 xy + 2 x + 2 y + 2 = 0. The equation of the asymptotes of the conjugate hyperbola should differ by the same constant. The equation of the asymptotes of the conjugate hyperbola is 2 xy + 2 x + 2 y + 1 = 0.

Example 8.7.6 Derive the equations of asymptotes.

Solution

x2 y2 − 2 = 1 (i.e.) f ( x1 , y1 ) = b 2 x 2 − 2 a b a 2 y 2 − a 2 b 2 = 0. This being a second-degree equation, it can have maximum two asymptotes. As the coefficients of the highest degree terms in x and y are constants, there is no asymptote parallel to the axes of coordinates. Take x = 1 and y = m in the highest degree terms f ( m) = b 2 − a 2 m2 . Similarly f ( m) = 0. The slopes of the oblique asymptotes are given by b f2 ( m) = 0. (i.e.) m = ± . a f ( m) 0 Also, c = − 1′ = =0 f 2 ( m) −2a 2 m The equation of the hyperbola is

The equations of the asymptotes are given by y = ± (i.e.)

b x. a

x y x y − = 0 and + = 0. Therefore, the combined equation is a b a b

x2 y2 − = 0. a2 b2

Exercises 1.

Prove that the tangent to the hyperbola x 2 − 3 y 2 = 3 at ( 3 , 0) when associated with the two asymptotes form an equilateral triangle whose area is 3 square units.

2.

Prove that the polar of any point on any asymptote of a hyperbola with respect to the hyperbola is parallel to the asymptote.

3.

Prove that the rectangle contained by the intercepts made by any tangent to a hyperbola on its asymptotes is constant.

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Analytical Geometry

4.

From any point of the hyperbola tangents are drawn to another which has the same asymptotes. Show that the chord of contact cuts off a constant area from the asymptotes.

5.

Find the equation of the hyperbola whose asymptotes are x + 2 y + 3 = 0 and 3 x + 4 y + 5 = 0 and which passes through the point (1, −1) Ans.: ( x + 2 y − 13)(3 x + 4 y + 3) − 8 = 0

 6. 

Find the asymptotes of the hyperbola 3 x 2 − 5 xy − 2 y 2 + 5 x + 11 y − 8 = 0. Ans.: x − 2 y + 3 = 0 3 x + y − 4 = 0

7.

Prove that the locus of the centre of the circle circumscribing the triangle x2 y2 formed by the asymptotes of the hyperbola 2 − 2 = 1 and a variable tangent a b  x2 y2  1 1 is 4  2 − 2  = 2 + 2 . a  a b b

8.

Find the equation of the asymptotes of the hyperbola 9 y 2 − 4 x 2 = 36 and obtain the product of the perpendicular distance of any point on the hyperbola 36 from the asymptotes. ay2 - 4x2 = 0; 13

9.

Show that the locus of the point of intersection of the asymptotes with the x2 y2 directrices of the hyperbola 2 − 2 = 1 is the circle x 2 + y 2 = a 2 . a b

10.

Let C be the centre of a hyperbola. The tangent at P meets the axes in Q and R and the asymptotes in L and M. The normal at P meets the axes in A and B. Prove that L and M lie on the circle OAB and Q and R are conjugate with respect to the circle.

11.

x y If a line through the focus S drawn parallel to the asymptotes − = 0 of 2 2 a b x y the hyperbola 2 − 2 = 1 meets the hyperbola and the corresponding a b directrix at P and Q then show that SQ = 2 ⋅ SP . x2 y2 − = 1 and show that the straight a2 b2 line parallel to an asymptote will meet the curve in one point at infinity.

12.

Find the asymptotes of the hyperbola

13.

Prove that the product of the intercepts made by any tangent to a hyperbola on its asymptotes is a constant.

14.

If a series of hyperbolas is drawn having a common transverse axis of length 2a then prove that the locus of a point P on each hyperbola, such that its distance from one asymptote is the curve ( x 2 − b 2 )2 = 4 x 2 ( x 2 − a 2 ) .

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8.8  conjugate diameters Locus of mid points of parallel chords of the hyperbola is Let (x1, y1) be the mid point of a chord of the hyperbola Then its equation is

8-35

x 2 y2 − = 1. a 2 b2

x2 y2 − =1. a2 b2

xx1 yy1 x12 y12 − 2 = 2 − 2 a2 b a b

The slope of this chord is

b 2 x1 a 2 y1

Let this chord be parallel to y = mx. Then m =

b 2 x1 a 2 y1



(i.e.) 

y1 =

b 2 x1 a2 m

b2 x The locus of (x1, y1) is y = 2 , which is a straight line passing through the am origin. b2 b2 If y = m′x bisects all chords parallel to y = mx then m ′ = 2 or mm ′ = 2 . am a By symmetry, we note that y = mx will bisect all chords parallel to y = m′x.

Definition 8.8.1  Two diameters are said to be conjugate if each bisects

chords parallel to the other. The condition of the diameters y = mx and y = b2 m′x to be conjugate diameters is mm ′ = 2 . a

Note 8.8.2  These diameters are also conjugate diameters of the conjugate hyperbola

x2 −b2 b2 y2 − = 1 since mm = = . ′ − a2 −b2 − a2 a2

Property 8.8.1 If a diameter meets a hyperbola in real points, it will meet the conjugate hyperbola in imaginary points and its conjugate diameter will meet the hyperbola in imaginary points and the conjugate hyperbola in real points and vice versa.

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Analytical Geometry

Proof Let the equation of the hyperbola be x2 y2 − = 1 a2 b2



(8.5)

Then the equation of the conjugate hyperbola is x2 y2 − = −1 a2 b2



(8.6)

Let y = mx and y = m′x be a pair of conjugate diameters of the hyperbola (8.5). Then mm ′ =



b2 a2

(8.7)

The points of intersection of y = mx and the hyperbola (8.5) are given by x 2 m2 x 2 − 2 = 1. a2 b

∴

x2 =

a2 b2 b2 − a2 m2

or x = ±

ab b − a2 m2 2



(8.8)

Since the hyperbola meets y = mx in real points from (8.8) b2 - a2 m2 > 0. The points of intersection of (8.6) with y = mx are given by

x2 = −

a2 b2 < 0 since b 2 − a 2 m2 > 0 b2 − a2

Therefore, y = mx meets the conjugate hyperbola in imaginary points. The points of intersection of y = m′x with the hyperbola (8.5) are given by

x2 = −

= =

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a2 b2 b − a2 m ′ 2 2

a2 b2  b2  b2 − a2  4 2  a m 

 b2  mm = ∵ ′  a 2 

a 4 m2 0 b − a2 m2 2

y = m′x meets the conjugate hyperbola in real points.

Property 8.8.2 If a pair of conjugate diameters meet the hyperbola and its conjugate hyperbola in P and D, respectively then CP2 - CD2 = a2 - b2.

Proof Let P be the point (a sec q, b tan q ) Then D will have coordinates (-a tan q , -b sec q ). Then CP2 = a2 sec2 q + b2 tan2 q CD2 = a2 tan2 q + b2 sec2 q CP2 - CD2 = a2 (sec2 q - tan2 q ) - b2 (sec2 q - tan2 q ) = a2 - b2

Property 8.8.3 The parallelogram formed by the tangents at the extremities of conjugate diameters of hyperbola has its vertices lying on the asymptotes and is of constant area.

Proof Let P and D be points (a sec q, b tan q ) and (a tan q, b sec q ) on the hyperbola and its conjugate.

D P P′ D′

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Analytical Geometry

Then D′ and P′ are (-a tan q , -b sec q ) and (-a sec q , –b tan q ), respectively. The equations of the asymptotes are

x y x y − = 0 and + = 0. a b a b

The equations of the tangents at P, P′, D, D′ are

−



−

x secq y tan q − = 1 a b

(8.9)

x secq y tan q + = 1 a b

(8.10)

x tan q y secq − = −1 a b

(8.11)

x tan q y secq + = −1 a b

(8.12)

respectively. Clearly the tangents at P and P′ are parallel and also the tangents at D and D′ are parallel. Solving (8.9) and (8.11) we get the coordinates of D are [a(sec q + tan q ), b[sec q + tan q )]. This lies on the asymptote

x y − = 0. a b

Similarly the other points of intersection also lie on the asymptotes. The equations of PCP′ and DCD′ are

y=

b tan q x a secq

(8.13)



y=

b secq x a tan q

(8.14)

Lines (8.11), (8.12) and (8.13) are parallel and also the lines (8.9), (8.10) and (8.11) are parallel. Therefore, area of parallelogram ABCD = 4 area of parallelogram CPAD.

= 4CP (Perpendicular from C on AD)



= 4 a 2 sec2 q + b 2 tan 2 q .



= 4 ab which is a constant.

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1 tan q sec2 q + a2 b2 2

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Hyperbola

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Example 8.8.1 If a pair of conjugate diameters meet hyperbola and its conjugate, respectively in P and D then prove that PD is parallel to one of the asymptotes and is bisected by the other asymptote.

Solution Let the equation of the hyperbola be x2 y2 − = 1 a2 b2 The equation of the conjugate hyperbola is x2 y2 − = −1 a2 b2 The asymptotes of the hyperbola (1) are x y − = 0 a b

(8.15)

(8.16)

(8.17)

x y + = 0 (8.18) a b Let P be the point (a sec q , b tanq ). Then D is the point (a tan q , b secq ). b(sec q − tan q ) b = − = The slope of the The slope of the chord PD is a (tan q − sec q ) a asymptote (8.18)

PD is parallel to the asymptote (8.18). b a  The midpoint of PD is a  (sec q + tan q ), (tan q + secq ) . This point 2  2 lies on the asymptotes given by (8.17). Therefore, PD is bisected by the other asymptote.

Example 8.8.2 In the hyperbola 16x2 - 9y2 =144 find the equation of the diameter conjugate to the diameter x = 2y.

Solution The equation of the hyperbola is 16x2 - 9y2 =144 x2 y2 (i.e.)  − =1 9 16

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a2 = 9,  b2 = 16

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Analytical Geometry

1 2 b2 If m and m′ are the slopes of the conjugate diameters then mm ′ = 2 a 1 16 32 or m ′ = m′ = 2 9 9 32 Therefore, the equation of the conjugate diameter is y = x or 32 x − 9 y = 0. 9 The slope of the line x = 2y is m =

Example 8.8.3 Find the condition that the pair of lines Ax2 + 2Hxy + By2 = 0 to be conjugate x2 y2 diameters of the hyperbola 2 − 2 = 1. a b

Solution Let the two straight lines represented by Ax2 + 2Hxy + By2 = 0 be y = m1x and y = m2x. Then

m1 + m2 = −

2H A and m1m2 = B B

(8.19)

If these lines are the conjugate diameters of the hyperbola then From (8.19) and (8.20) or

m1m2 =

b2 a2

(8.20)

A b2 = B a2 a2A = b2B.

Property 8.8.4 Any two conjugate diameters of a rectangular hyperbola are equally inclined to the asymptotes.

Proof Let the equation of the rectangular hyperbola be x2 - y2 = a2. The equation of the asymptotes is x2 - y2 = 0. 1 Let y = mx and y = x be a pair of conjugate diameters of the rectangular m hyperbola be x2 - y2 = a3.

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Then the combined equation of the conjugate diameters is (y - mx) (x - my) = 0



(i.e.) 

mx2 - (m2 + 1)xy + my2 = 0.

The combined equation of the bisectors of the angles between these two lines is



x2 − y2 − xy = 1 2 0 ( m + 1) 2 (i.e.)  x2 - y2 = 0.

This is the combined equation of the asymptotes. Therefore, the asymptotes bisect the angle between the conjugate diameter.

8.9  RECTANGULAR HYPERBOLA Definition 8.9.1 If in a hyperbola the length of the semi-transverse axis is equal to the length of the semi-conjugate axis, then the hyperbola is said to be a rectangle hyperbola.

8.9.1  Equation of Rectangular Hyperbola with Reference to Asymptotes as Axes y

A 45° C L

x

A′

In a rectangular hyperbola, the asymptotes are perpendicular to each other. Since the axes of coordinates are also perpendicular to each other, we can take the asymptotes as the x- and y-axes. Then the equations of the asymptotes are x = 0 and y = 0. The combined equation of the asymptotes is xy = 0.

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Analytical Geometry

The equation of the hyperbola will differ from that of asymptotes only by a constant. Hence, the equation of the rectangular hyperbola is xy = k where k is a constant to be determined. Let AA′ be the transverse axis and its length be 2a. Then, AC = CA′ = a. Draw AL perpendicular to x-axis. Since the ­asymptotes bisect the angle between the axes, ACL = 45°.

CL = CA cos 45° =

a 2

and AL = CA sin 45° =

a 2

 a a  The coordinates of A are  , . Since it lies on the rectangular hyper 2 2 2 a bola xy = k , we get = k . Hence, the equation of the rectangular hyperbola 2 2 a a2 is xy = or xy = c 2 where c 2 = . 2 2

Note 8.9.1.1:  The parametric equations of the rectangular hyperbola c xy = c 2 are x = ct and y = . t

8.9.2  Equations of Tangent and Normal at (x1, y1) on the Rectangular Hyperbola xy = c 2 The equation of rectangular hyperbola is xy = c 2 . Differentiating with respect to x, we get dy x +y=0 dx dy − y = dx x −y dy = 1 = slope of the tangent at( x1 , y1 ). dx at ( x1 , y1 ) x1 The equation of the tangent at ( x1 , y1 ) is y − y1 =



− y1 ( x − x1 ) x1

⇒ x1 y − x1 y1 = − y1 x + x1 y1

⇒ y1 x + x1 y = 2 x1 y1 (i.e.)

y1 x + x1 y = 2c 2

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Hyperbola

x1 y1 = c 2.

since

The slope of the normal at ( x1 , y1 ) is

x1 . y1

The equation of the normal at ( x1 , y1 ) is y − y1 =



x1 ( x − x1 ) y1

yy1 − y12 = xx1 − x12 (i.e.)

xx1 − yy1 = x12 − y12

 c 8.9.3  Equation of Tangent and Normal at  ct ,  on the Rectangular  t Hyperbola xy = c 2 The equation of the rectangular hyperbola is xy = c 2 . Differentiating with respect to x, we get dy x +y=0 dx dy − y ∴ = dx x dy  c  −1  c  ct ,  = 2 = slope of the tangent at  ct ,  dx t t t  c The equation of the tangent at is  ct ,  is  t y−



(i.e.)

c −1 = ( x − ct ) t t2

yt 2 − ct = − x + ct or x + yt 2 = 2ct

The slope of the normal at ‘t’ is −t 2 . The equation of the normal at ‘t’ is Dividing by t, we get

c = −t 2 ( x − ct ) t c y − = − xt 2 + ct 3 t y−

y c − = − xt + ct 2 t t2

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or

xt −

y 1  = c t2 − 2   t t 

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Analytical Geometry

8.9.4  Equation of the Chord Joining the Points ‘t1’ and ‘t2’ on the Rectangular Hyperbola xy = c 2 and the Equation of the Tangent at t   c c The two points are  ct1 ,  and  ct2 ,  . The equations of the chord joining t t    1 2 the two points are y − y1 y1 − y2 = x − x1 x1 − x2 c c c − y− −c(t1 − t2 ) t t t1 −1 = 1 2 = = x − ct1 ct1 − ct2 ct1t2 (t1 − t2 ) t1t2 Cross multiplying, we get yt1t2 − ct2 = − x + ct1

x + yt1t2 = c(t1 + t2 )

This chord becomes the tangent at ‘t’ if t1 = t2 = t . Hence, the equation of the tangent at ‘t’ is x + yt 2 = 2ct .

8.9.5  Properties Any two conjugate diameters of a rectangular hyperbola are equally inclined to the asymptotes.

Proof  Let the equation of the rectangular hyperbola be x 2 − y 2 = a2 . The

1 x be a pair m 2 2 2 of conjugate diameters of the rectangular hyperbola x − y = a . Then, the combined equation of the conjugate diameters is ( y − mx )( x − my ) = 0 (i.e.)  mx 2 − ( m2 + 1) xy + my 2 = 0 The combined equation of the bisectors of the angles between these two lines 2 2 is x − y = − 2xy (i.e.) x 2 − y 2 = 0 which is the combined equation of 1 0 2 ( m + 1) the asymptotes. Therefore, the asymptotes bisect the angle between the conjugate diameter. equation of the asymptotes is x 2 − y 2 = 0 . Let y = mx and y =

8.9.6  Results Concerning the Rectangular Hyperbola 1. The equation of the tangent at ( x1 , y1 ) on the rectangular hyperbola xy = c 2 1 is ( xy1 + yx1 ) = c 2 . 2

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2. The equation of the normal at ( x1 , y1 ) is xx1 − yy1 = x12 − y12 . 3. The equation of the pair of tangents from ( x1 , y1 ) is ( xy1 + yx1 − 2c 2 )2 = 4( xy − c 2 )( x1 y1 − c 2 ). 4. The equations of the chord having ( x1 , y1 ) as its midpoint is xy1 + yx1 = 2 x1 y1 . 5. The equation of the chord of contact from ( x1 , y1 ) is xy1 + yx1 = 2c 2 .

8.9.7  Conormal Points—Four Normal from a Point to a Rectangular Hyperbola Let ( x1 , y1 ) be a given point and t be the foot of the normal from ( x1 , y1 ) on the rectangular hyperbola xy = c 2 . 1 y  The equation of the normal at t is xt − = c  t 2 − 2  .  t t  Since this normal passes through ( x1 , y1 ), x1t −

(i.e.)

y1 1  = c t2 − 2   t t 

x1t 3 − y1t = c(t 4 − 1) or ct 4 − x1t 3 + y1t − c = 0

This is a fourth-degree equation in t and there are four values of t(real or imaginary). Corresponding to each value of t there is a normal, and hence there are four normals from a given point to the rectangular hyperbola.

Note 8.9.7.1:  If t1 , t2 , t3 and t 4 are the four points of intersection, then ∑ t1 =

x1 −y , ∑ t1t2 = 0, ∑ t1t2 t3 = and t1t2 t3t 4 = −1. c c

8.9.8  Concyclic Points on the Rectangular Hyperbola Let the equation of the rectangular hyperbola be xy = c 2 . Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + k = 0.  c Let  ct ,  be a point of intersection of rectangular hyperbola and the cir t c  c cle. Then, the point  ct ,  also lies on the circle. Substituting x = ct , y =  t t in the equation of the circle we get

c2t 2 +

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c2 c + 2 gct + 2 f ⋅ + k = 0 2 t t

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Analytical Geometry

(i.e.)  c 2 t 4 + 2 gct 3 + kt 2 + 2 fct + c 2 = 0



This is a fourth degree equation in t. For each value of t, there is a point of intersection (real or imaginary). Hence, there are four points of intersection for a rectangular hyperbola with the circle.

Note 8.9.8.1:  If t1 , t2 , t3 and t 4 are the four points of intersection, then ∑ t1 =

−2a k −2 f , ∑ t1 t2 = 2 , ∑ t1 t2 t3 = and t1 t2 t3 t 4 = 1. c c c

Example 8.9.1 Show that the locus of poles with respect to the parabola y 2 = 4ax of tangents to the hyperbola x 2 − y 2 = a 2 is the ellipse 4 x 2 + y 2 = 4 a 2 .

Solution Let ( x1 , y1 ) be the pole with respect to the parabola y 2 = 4 ax . Then, the polar 2ax 2ax1 + . y1 y1 This is a tangent to the rectangular hypherbola x 2 − y 2 = a 2 . The condition for tangency is of ( x1 , y1 ) is yy1 = 2a( x + x1 ) (i.e.) y =

c 2 = a 2 ( m2 − 1)





2 2    2ax1  2  2a   = a − 1 (i.e.)      y1   y1   2 2 2 2 2 4 a x1 = a ( 4 a − y1 ) (i.e.)



4 x12 + y12 = 4 a 2

The locus of ( x1 , y1 ) is 4 x 2 + y 2 = 4 a 2 which is an ellipse.

Example 8.9.2 P is a point on the circle x 2 + y 2 = a 2 and PQ and PR are tangents to the hyperbola x 2 − y 2 = a 2 . Prove that the locus of the middle point of QR is the curve ( x 2 − y 2 )2 = a 2 ( x 2 + y 2 ).

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Solution Let P ( x1 , y1 ) be a point on the circle x 2 + y 2 = a 2 . ∴ x12 + y12 = a 2



Since PQ and PR are tangents from P to the rectangular hyperbola x 2 − y 2 = a 2 , QR is the chord of contacts of tangents from P ( x1 , y1 ). Therefore, its equation is xx1 + yy1 = a 2 . Let ( h, k ) be the midpoint of QR. Its equation is xh − yk = h2 − k 2 . These two equations represent the same line. Therefore, identifying them, we get

x1 y1 a2 = = 2 h k h − k2



x1 =

Since x12 + y12 = a 2 ,

a2 h a2 k and y1 = 2 2 h − k2 h −k 2

a4 ( h2 + k 2 ) = a 2 (i.e.) a 2 ( h2 + k 2 ) = ( h2 − k 2 )2 . ( h2 − k 2 ) 2

The locus of ( h, k ) is ( x 2 − y 2 )2 = a 2 ( x 2 + y 2 ).

Example 8.9.3 Prove that the locus of poles of all normal chords of the rectangular hyperbola xy = c2 is the curve (x2 - y2) + 4c2xy = 0.

Solution Let ( x1 , y1 ) be the pole of the normal chord of rectangular hyperbola xy = c 2 . The poles of ( x1 , y1 ) is xy1 + yx1 = 2c 2 . Let the chord be normal at t. 1 y  The equation of the normal at t is xt − = c  t 2 − 2  . These two equations  t t  represent the same straight line. Identifying them, we get



y1 2c 2 = −tx1 = 1 t  c t2 − 2   t  ∴t 2 =

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− y1 x1

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Analytical Geometry

x12 t12 =

Also

4c 2  2 1  t − 2  t

2

2

or

2

 −y   −y x  1  x t  t 2 − 2  = 4c 2 (i.e.) x12  1   1 + 1  = 4c 2   y1  t  x1   x1 2 2 1 1

( x12 − y12 )2 + 4c 2 x1 y1 = 0

The locus of ( x1 , y1 ) is ( x 2 − y 2 )2 + 4c 2 xy = 0.

Example 8.9.4 If P is any point on the parabola x 2 + 16ay = 0, prove that the poles of P with respect to rectangular hyperbola xy = 2a 2 will touch the parabola y 2 = ax .

Solution Let ( x1 , y1 ) be any point. The polar of P with respect to the hyperbola is −y 4a2 . This is a tangent to the parabola xy1 + x1 y = 4 a 2 (i.e.) y = 1 x + x1 x1 a y 2 = ax . The condition is c = . 4m 2 4a a = (i.e.) 16ay1 + x12 = 0 (i.e.)  x1  − y1  4  x1  The locus of ( x1 , y1 ) is x 2 + 16ay = 0.

Example 8.9.5 A tangent to the parabola x 2 = 4 ay meets the hyperbola xy = c 2 at P and Q. Prove that the middle point of PQ lies on a fixed parabola.

Solution Let ( x1 , y1 ) be the midpoint of the chord PQ of the rectangular hyperbola xy = c 2 . The equation of chord PQ is xy1 + yx1 = 2 x1 y1 − yx1 (i.e.) xy1 = − yx1 + 2 x1 y1 ⇒ x = + 2 x1 y1

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This is a tangent to the parabola x 2 = 4 ay. Therefore, the condition is a . 2 x1 =  − x1   y  1 (i.e.) 2 x12 + ay1 = 0 . The locus of ( x1 , y1 ) is 2 x 2 + ay = 0, which is a fixed parabola.

Example 8.9.6 Find the locus of midpoints of chords of constant length 2l of the rectangular hyperbola xy = c 2 .

Solution Let R( x1 , y1 ) be the midpoint of the chord PQ. Let the equation of the chord x − x1 y − y1 = = r . PQ be cos q sin q y P R (x1, y1) Q

x

Any point on this line is x = x1 + r cos q , y = y1 + r sin q . If this point lies on the rectangular hyperbola xy = c 2 , we get ( x1 + r cos q )( y1 + r sin q ) = c 2.

r 2 sin q cos q + r ( x1 sin q + y1 cos q ) + x1 y1 − c 2 = 0

(8.21)

This is a quadratic equation in r. The two values of r are the distances RP and RQ which are equal in magnitude but opposite in sign. The condition for this is the coefficient of r is equal to zero. x1 sin q + y1 cos q = 0



(8.22)

Then, equation (8.21) becomes

r 2 sin q cos q + x1 y1 − c 2 = 0

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(8.23)

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Analytical Geometry

From equation (8.22), tan q =

− y1 ,sin q = x1

−y x +y 2 1

2 1

and cos q =

x1 x + y12 2 1

 −x y  Substituting these in equation (8.23), we get r 2  2 1 12  + x1` y1 − c 2 = 0.  x1 + y1  ( x12 + y12 )( x1 y1 − c 2 ) − r 2 x1 y1 = 0 ; but r = l . Therefore, the locus of ( x1 , y1 ) is ( x 2 + y 2 )( xy − c 2 ) − l 2 xy = 0 .

Example 8.9.7 If PP′ is a diameter of the rectangular hyperbola xy = c 2 show that the locus of the intersection of tangents at P with the straight line through P ′ parallel to either asymptote is xy + 3c 2 = 0 .

Solution

−c   c  Let P be the point  ct ,  . Then P ′ is the point  −ct ,  .  t  t  The equation of the tangent at P is x + yt 2 = 2ct . y

P C

P′

x R

−c . t Let ( x1 , y1 ) be the point of intersection of these two lines. Then The equation of the straight line P ′R parallel to x-axis is y =



x1 + y1t 2 = 2ct −c t −c (i.e.)  t = y1

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y1 =

(8.24) (8.25)

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Hyperbola

Substituting in equation (8.24), x1 + y1 ⋅ x1 +



 −c  c2 = 2c   2 y1  y1 

c 2 −2c 2 = y1 y1

xy1 + c 2 = −2c 2



8-51

or

x1 y1 + 3c 2 = 0

The locus of ( x1 , y1 ) is xy + 3c 2 = 0.

Example 8.9.8 The tangents to the rectangular hyperbola xy = c 2 and the parabola y 2 = 4 ax at their point of intersections are inclined at angles a and b , respectively, to the x-axis. Show that tan a + 2 tan b = 0.

Solution Let ( x1 , y1 ) be the point of intersection of the rectangular hyperbola xy = c 2 and the parabola y 2 = 4 ax . The equation of tangent at ( x1 , y1 ) to the parabola is yy1 = 2a( x + x1 ). The equation of tangent to the rectangular hyperbola is xy1 + yx1 = 2c 2 . 2a . The slope of the tangent to the parabola is tan b = y1 The slope of the tangent to the tangent to the rectangular hyperbola is −y tana = 1 . x1 tan a + 2 tan b =

− y1 2a − y12 + 4 ax1 + 2⋅ = =0 x1 y1 x1 y1

since ( x1 , y1 ) lies on the parabola y 2 = 4 ax . ∴ tan a + 2 tan b = 0

Example 8.9.9 If the normal to the rectangular hyperbola xy = c 2 at the point t as it intersect the rectangular hyperbola at t1 then show that t 3t1 = −1.

Solution The equation of the normal at t is xt −

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1 y  = c t2 − 2 .  t t 

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Analytical Geometry

y

P (t) x

0 t1

The equation of the chord joining the points t and t1 is x + y t t1 = c(t + t1 ). These two equations represent the same straight line. Identifying them, we get



1  c t2 − 2   t −1/t t  = = 1 tt1 c (t + t1 ) ∴t =

−1 or t 3t1 = −1 t 2 t1

Example 8.9.10 Show that the area of the triangle formed by the two asymptotes of the rectangular hyperbola xy = c 2 and the normal at ( x1 , y1 ) on the hyperbola is 2 1  ( x12 − y12 )   . c 2 

Solution The equation of the normal at ( x1 , y1 ) is xx1 − yy1 = x12 − y12 . y

t A

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B 0

x

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When the normal meets the x-axis, y = 0. ∴x =



x12 − y12 = OA x1

When the normal meets y-axis, x = 0 ∴y =



−( x12 − y12 ) = OB y1

The area of the triangle OAB = ∆= =

1 ⋅ OA ⋅ OB 2

−1 x12 − y12 x12 − y12 ⋅ ⋅ x1 y1 2 −1 ( x12 − y12 )2 ⋅ x1 y1 2

2

(i.e.)  ∆ =

1  ( x12 − y12 )  2   since x1 y1 = c and ignoring the negative sign. c 2 

Example 8.9.11 If four points be taken on a rectangular hyperbola such that the chord joining any two is perpendicular to the chord joining the other two and a , b , g, d are the inclinations of the straight lines joining these points to the centre. prove that tan a tan b tan g tan d = 1.

Solution Let t1 , t2 , t3 , and t 4 be four points P , Q, R, and S on the rectangular hyperbola xy = c 2 . The equation of the chord joining t1 and t2 is x + y t1t2 = c(t1 + t2 ). −1 . The slope of this chord is t1t2 −1 Similarly, the slope of the chord joining t3 and t 4 is . t3t 4  −1   −1  = −1. Since these two chords are perpendicular,   t1t2   t3t 4 

(i.e.) t1 t2 t3 t 4 = − 1

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(8.26)

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Analytical Geometry

(c /t1 ) 1 (i.e.) tana = 2 c t1 t1 1 1 1 Similarly, tan b = 2 , tan g = 2 , tan d = 2 t2 t3 t4 1 ∴ tan a ⋅ tan b ⋅ tan g ⋅ tan d = 2 2 2 2 = 1 from equation (8.26) t1 ⋅ t2 ⋅ t3 ⋅ t 4 The slope of the line CP is tan a =

Example 8.9.12 If the normals at three parts P , Q and R on a rectangular hyperbola intersect at a point S on the curve then prove that the centre of the hyperbola is the centroid of the triangle PQR.

Solution If the normal at t meets the curve at t ′ then t 2 t ′ = −1. (i.e.)  t 2 t ′ + 1 = 0



(8.27)

This is a cubic equation in t. If t1 , t2 and t3 are the roots of this equation they can be regarded as the parameters of the points P , Q and R, the normals at these points meet at t ′which is S. From equation (8.27), we get t1 + t2 + t3 = 0 and t1t2 + t2 t3 + t3t1 = 0. Let ( h, k ) be the centroid of ∆PQR. Then h =

x1 + x2 + x3 c(t1 + t2 + t3 ) = = 0. 3 3

y1 + y2 + y3 c  1 1 1  c  t1t2 + t2 t3 + t3t1  =  + + =   = 0. t1t2 t3 3 3  t1 t2 t3  3  The centroid is the centre of the rectangular hyperbola.



k=

Example 8.9.13 Show that four normals can be drawn from a point ( h, k ) to the rectangular hyperbola xy = c 2 and that its feet form a triangle and its orthocentre.

Solution The equation of the normal at t is xt −

1 y  = c t2 − 2 .  t t 

(i.e.)  ct 4 − xt 3 + yt − c = 0

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Since this passes through ( h, k ) , ct 4 − ht 3 + kt − c = 0. This is a fourth degree equation in t. Its roots are t1 , t2 , t3 and t 4 which are the feet of the four normals from ( h, k ) . ∴ t1 + t2 + t3 + t 4 =



h −k , ∑ t1t2 = 0, ∑ t1t2 t3 = , t1t2 t3t 4 = −1 c c

If t1 , t2 , t3 and t 4 are the points P , Q, R and S on the rectangular hyperbola xy = c 2 , it can be shown that the orthocentre of the triangle is   −c  t t t , − ct1 t2 t3  . 1 2 3

 c This point is  t 4 ,  since t1t2 t3t 4 = −1.  t4  ∴ The four points P , Q, R and S form a triangle and its orthocentre.

Example 8.9.14 Prove that from any point ( h, k ) four normals can be drawn to the rectangular hyperbola xy = c 2 and that if the coordinates of the four feet of the normals P , Q, R and S be ( xr , yr ), r = 1, 2, 3, 4. Then (i) x1 + x2 + x3 + x4 = h, y1 + y2 + y3 + y4 = k and (ii) x1 x2 x3 x4 = y1 y2 y3 y4 = −c 4.

Solution The equation of the normal at t is xt −



1 y  = c t2 − 2   t t 

Since this passes through ( h, k )

ht −

1 k  = c  t 2 − 2  (i.e.) ct 4 − ht 3 + kt − c = 0.  t t 

The form values of t correspond to the feet of the four normals from the point ( h, k ) . If t1 , t2 , t3 and t 4 are the four feet of the normals then they are the roots of the above equation. h t1 + t2 + t3 + t 4 = (8.28) c

∑t t



∑t t t

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1 2

1 2 3

= 0 =

−k c

(8.29) (8.30)

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Analytical Geometry

t1t2 t3t 4 = −1



(8.31)

From equation (8.28), c(t1 + t2 + t3 + t 4 ) = h (i.e.)  x1 + x2 + x3 + x4 = h



Dividing equation (8.30) by equation (8.31), we get c c c c + + + =k t1 t2 t3 t 4 (i.e.)  y1 + y2 + y3 + y4 = k



x1 x2 x3 x4 = c 4 t1t2 t3t 4 = −c 4

Also, y1 y2 y3 y4 =



1 1 1 1 k + + + = . t1 t2 t3 t 4 c

c4 = −c 4 , x1 x2 x3 x4 = y1 y2 y3 y4 = −c 4 t1t2 t3t 4

Example 8.9.15 Prove that the feet of the concurrent normals on the rectangular hyperbola xy = c 2 which meets at ( h, k ) lie on another rectangular hyperbola which passes through (0, 0) and ( h, k ) .

Solution The equation of the normal at ( x1 , y1 ) is xx1 − yy1 = x12 − y12 . Since this passes through ( h, k ) , hx1 − ky1 = x12 − y12 The locus of ( x1 , y1 ) is x 2 − y 2 − hx + ky = 0. Clearly this is a rectangular hyperbola passing through (0, 0) and ( h, k ) .

Example 8.9.16 If a rectangular hyperbola whose centre is c is cut by any circle of radius r in four points P , Q, R, S then prove that CP 2 + CQ 2 + CR2 + CS 2 = 4 r 2 .

Solution Let the equation of the rectangular hyperbola be xy = c 2

(8.32)

x 2 + y 2 + 2 gx + 2 fy + k = 0

(8.33)

Let the equation of the circle be

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Hyperbola

c2 Solving these two equations, we get their points of intersections y = . x c4 c2 Substituting in equation (8.33), x 2 + 4 + 2 gx + 2 f + k = 0. x x (i.e.)  x 4 + 2 gx 3 + kx 2 + 2 fc 2 x + c 4 = 0.



If x1 , x2 , x3 , x4 are the abscissae of the four points of intersection x1 + x2 + x3 + x4 = −2 g .

∑x x

1 2

= k , x12 + x22 + x32 + x42 = ( x1 + x2 + x3 + x4 )2 − 2∑ x1 x2 = 4 g 2 − 2k .

Similarly, y12 + y22 + y32 + y42 = 4 f 2 − 2k =  CP 2 + CQ 2 + CR2 + CS 2

= ∑ ( x12 + y12 ) = ∑ x12 + y12 = 4( g 2 + f 2 − k ) = 4 r 2

Example 8.9.17 A, B, C and D are four points of intersection of a circle and a rectangular hyperbola. If AB passes through the centre of the hyperbola, show that CD passes through the centre of the circle.

Solution Let the equation of the rectangular hyperbola be xy = c 2 . Let the equation of the circle be x 2 + y 2 + 2 gx + 2 fy + k = 0. Let A, B, C and D be the points t1 , t2 , t3 and t 4, respectively. When the circle and rectangular hyperbola intersect we know that −2 g (8.34) t1 + t2 + t3 + t 4 = c

∑t t

=

k c2

(8.35)



∑t t t

=

−2 f c

(8.36)



1 2

1 2 3

t1t2 t3t 4 = 1

(8.37)

The equation of the chord AB is x + yt1t2 = c(t1 + t2 ). Since AB passes through (0, 0), we get

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t1 + t2 = 0

(8.38)

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8-58

Analytical Geometry

∴ From equation (8.34), t3 + t 4 = −

−2 f c

However,

∑t t t



t1t2 t3 + t1t2 t 4 + t1t3t 4 + t2 t3t 4 =

1 2 3

=

2g c

(8.39)

−2 f c −2 f . t1t2 (t3 + t 4 ) + (t1 + t2 )t3t 4 = c



1 −2 f (t 3 + t 4 ) + 0 = using equation (8.38) t3t 4 c −2 f t3 + t 4 = (t3t 4 ) c  −2 g   −c  g = [From equations (8.39) and (8.40)] t3t 4 =   c   2 f  h

(8.40)

The equation of the chord CD is x + yt3t 4 = c(t3 + t 4 ).  g x + y   = −2 g  f



This straight line passes through the point ( − g , − f ). Therefore, CD passes through the centre of the circle.

Example 8.9.18 Show that through any given point P in the plane of xy = c 2 , four normals can be drawn to it. If P1 , P2 , P3 and P4 are feet of these normals and C is centre then show that CP12 + CP22 + CP32 + CP42 = CP 2 .

Solution

1 y  = c t2 − 2  .  t t  Let p be the point ( h, k ) . Since the normal passes through ( h, k ) , The equation of the normal at t is xt −

ht −

1 k  = c  t 2 − 2  (i.e.) ct 4 − ht 3 + kt − c = 0.  t t 

This brings a fourth degree equation, there are four normals from P.

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Hyperbola

If t1 , t2 , t3 and t 4 are the feet of the normals then t1 + t2 + t3 + t 4 = −k , t1t2 t3t 4 = −1. c

∑t t

= 0, ∑ t1t2 t3 =

Also,

1 1 1 1 , , , are the roots of ct 4 − kt 3 + ht − c = 0. t1 t2 t3 t 4

1 2

(

∑t

1

=

h . c

1 k ,∑ = 0. c t1t2

)

2  t1 − 2∑ t1 t2 +  ∑     1 1  ∑ CP 2 = ∑ c2  t12 + t 2  = c2 ∑ t12 + ∑ t 2  = c2  1  2 1   1 1  2 − ∑t t   ∑ t1  1 2  2 2  h   k   = c 2    +    = h2 + k 2  c   c  

= CP 2

Example 8.9.19 The slopes of the sides of triangle ABC inscribed in a rectangular hyperbola xy = c 2 are tan a , tan b and tan g . If the normals at A, B and C are concurrent show that cot 2a + cot 2 b + cot 2g = 0.

Solution Let A, B, C be the points t1 , t2 and t3 respectively. 1 The slope of AB is − . t1t2

∴ tana = −

∑ cot 2a

1 t1t2

=∑ =

1 − t12 t22 1 − t12 t22 1 − t22 t32 1 − t32 t12 1 − tan 2 a + =∑ = + 2 tan a 2t1t2 2t1t2 2t2 t3 2t3t1

t1 + t2 + t3 − t1t2 t3 (t1t2 + t2 t3 + t3t1 ) 2t1t2 t3

t 4 (t1 + t2 + t3 ) − t1t2 t3t 4 (t1t2 + t2 t3 + t3t1 ) 2t1t2 t3t 4 t 4 (t1 + t2 + t3 ) + (t1t2 + t2 t3 + t3t1 ) = −2 =

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8-60

Analytical Geometry

t1t2 =0 −2 ∴ cot 2a + cot 2 b + cot 2g = 0

=∑



Example 8.9.20 Show that an infinite number of triangles can be inscribed in a rectangular hyperbola xy = c 2 whose sides touch the parabola y 2 = 4 ax.

Solution Let ABC be a triangle inscribed in the rectangular hyperbola xy = c 2 . Let A, B and C be the points t1 , t2 , and t3, respectively. Suppose the sides AB and AC touch the parabola y 2 = 4 ax. The equation of the chord AB is x + yt1t2 = c(t1 + t2 ). This touches the parabola y 2 = 4 ax.  (i.e.) 

c(t1 + t2 ) = − at1t2 t1t2

(i.e.)  c(t1 + t2 ) + a(t1t2 )2 = 0 (i.e.)  at12 t22 + c(t1 + t2 ) = 0. Since AC also touches the parabola,

at12 t32 + c(t1 + t3 ) = 0

From these equations, we note that t2 , t3 are the roots of the equation at12 t 2 + ct + ct1 = 0 . t 2 + t3 = − t 2 t3 =

c at12

c at1

The equation of the chord BC is x + yt2 t3 = c(t2 + t3 ).



(i.e.)  x +

−c 2 c y= 2 at1 at1

(i.e.) 

y=

− at1 c x− c t1

This equation shows that BC touches the parabola y 2 = 4 ax. Since ABC is an arbitrary triangle inscribed in the rectangular hyperbola xy = c 2 there are infinite number of such triangles touching the parabola y 2 = 4 ax.

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8-61

Hyperbola

Exercises 1.

Prove that the portion of the tangent intercepted between by its asymptotes is bisected at the point of contact and form a triangle of contact area.

2.

If the tangent and normal to a rectangular hyperbola make intercepts a1 and a2 on one asymptote and b1 and b2 on the other then show that a1a2 + b1b2 = 0.

3.

2 P and Q are variable points on the rectangular hyperbola xy = c such that the tangent at Q passes through the foot of the ordinate of P. Show that the locus of the intersection of the tangents at P and Q is a hyperbola with the same asymptotes as the given hyperbola.

4.

If the lines x − a = 0 and y − b = 0 are conjugate lines with respect to the hyperbola xy = c 2 then prove that the point (a , b ) is on the hyperbola xy − 2c 2 = 0.

5.

If the chords of the hyperbola x 2 − y 2 = a 2 touch the parabola y 2 = 4 ax then prove that the locus of their middle points is the curve y 2 ( x − a) = x 3 .

6.

If PQ and PR are two perpendicular chords of the rectangular hyperbola xy = c 2 then show that QR is parallel to the normal at P .

7.

If the polar of a point with respect to the parabola y 2 = 4 ax touches the parabola x 2 = 4by , show that the point should lie on a rectangular hyperbola.

8.

 c Show that the normal at the rectangular hyperbola xy = c 2 at the point  ct ,  t  −c  meets the curve again at the point  3 , − ct 3  . Show that PQ varies as CP 2 t  where C is the centre.

9.

If PQ is a chord of the rectangular hyperbola xy = c 2 which is the normal at P show that 3CP 2 + CQ 2 = PQ 2 where C is the centre of the conic.

10.

Two rectangular hyperbolas are such that the axes of one are along the asymptotes of the other. Find the distance between the point of contact of a common tangent to them.

11.

Prove that any line parallel to either of the asymptotes of a hyperbola should meet it in one point at infinity.

12.

The tangent at any point of the hyperbola meets the asymptotes at Q and R. Show that CQ ⋅ CR is a constant.

13.

Prove that the locus of the centre of the circle circumscribing the triangle x2 y2 formed by the asymptotes of the hyperbola 2 − 2 = 1 and a variable tangent a b is 4( a 2 x 2 − b 2 y 2 ) = a 2 + b 2 .

14.

Show that the coordinates of the point of intersection of two tangents to a rectangular hyperbola are harmonic means between the coordinates of the point of contact.

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8-62

Analytical Geometry

15.

If the normals at A, B, C and D to the rectangular hyperbola xy = c 2 meet in P ( h, k ) then prove that PA2 + PB 2 + PC 2 + PD 2 = 3( h2 + k 2 ).

16.

If (c tan f , c cot f ) be a point on the rectangular hyperbola xy = c 2 then show that the chords through the points f and f ′ where f + f ′ is a constant passes through a fixed point on the conjugate axis of the hyperbola.

17.

Prove that the poles with respect to the circle x 2 + y 2 = a 2 of any tangent to the rectangular hyperbola xy = c 2 lies on rectangular hyperbola 4c 2 xy = c 2 .

18.

If a normal to a rectangular hyperbola makes an acute angle q with its transverse axis then prove that the acute angle at which it cuts the curve again is cot −1 (2 tan 2 q ).

19.

If a circle cuts the rectangular hyperbola xy = c 2 in four points then prove that the product of the abscissae of the points is c 4 .

20.

Let the rectangular hyperbola xy = c 2 is cut by a circle passing through its centre C in four points P , Q , R and S . If p, q be the perpendiculars from c on PQ , RS then show that pq = c 2 .

21.

If a triangle is inscribed in a rectangular hyperbola xy = c 2 and two of its sides are parallel to y = m1 x and y = m2 x then prove that the third side touches the hyperbola 4 m1m2 xy = c 2 ( m1 + m2 )2 .

22.

If a circle cuts the rectangular hyperbola xy = c 2 in P , Q , R and S and the parameters of these four points be t1 , t2 , t3 and t 4 , respectively then prove that the centre of the mean position of these points bisect the distance between the centres of the two curves.

23.

If three tangents are drawn to the rectangular hyperbola xy = c 2 at the points ( xi , yi ), i = 1, 2, 3 and form a triangle whose circumcircle passes through the ∑ x1 + ∑ y1 = 0 and that the centre centre of the hyperbola then show that x y1 y2 y2 1 x2 x3 of the circle lies on the hyperbola.

24.

If a circle with fixed centre (3 p, 3q) and of variable radius cuts the rectangular hyperbola x 2 − y 2 = 9c 2 at the points P , Q , R and S then show that the locus of the centroid of the triangle PQR is given by ( x − 2 p)2 − ( y − 2q)2 = a 2 .

25.

Show that the sum of the eccentric angles of the four points of intersection of an ellipse and a rectangular hyperbola whose asymptotes are parallel to the axes of the ellipse is an odd multiple of p .

26.

If from any point on the line lx + my + 1 = 0 tangents PQ , PR are drawn to the rectangular hyperbola 2 xy = c 2 and the circle PQR cuts the hyperbola again in T and T ′ then prove that TT ′ touches the parabola (l 2 + m2 )( x 2 + y 2 ) = (lx + my + 1)2 .

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Hyperbola

8-63

27.

If a circle cuts two fixed perpendicular lines so that each intercept is of given length then prove that the locus of the centre of the circle is a rectangular hyperbola.

28.

If A and B are points on the opposite branches of a rectangular hyperbola. The circle on AB as diameter cuts the hyperbola again at C and D then prove that CD is a diameter of the hyperbola.

29.

If A, B and C are three points on the rectangular hyperbola xy = d 2 whose abscissae are a, b and c respectively then prove that the area is 1 d 2 ( a − b)(b − c)(c − a) . and the area of the triangle enclosed by tangents at 2 abc 2 these points is 2d ( a − b)(b − c)(c − a) . ( a + b)(b + c)(c + a)

30.

If four points on a rectangular hyperbola xy = c 2 lie on a circle, then prove that the product of their abscissae is c 4 .

31.

If x1 , x2 , x3 and x4 be the abscissae of the angular points and the orthocentre of a triangle inscribed in xy = c 2 then prove that x1 x2 x3 x4 = − c 4 .

32.

Show that the length of the chord of the rectangular hyperbola xy = c 2 which

33.

( h2 + k 2 )( hk − c 2 ) . hk Prove that the point of intersection of the asymptotes of a rectangular hyperbola with the tangent at any point P and of the axes with the normal at P are equidistant from P. is bisected at the point ( h, k ) is 2

34.

If P is any point on a rectangular hyperbola whose vertices are A and A′ then prove that the bisectors of angle APA′ are parallel to the asymptotes of the curve.

35.

Let QCQ′ is a diameter of a rectangular hyperbola and P is any point on the curve. Prove that PQ , PQ ′ are equally inclined to the asymptotes of the hyperbola.

36.

Through the point P (0, b) a line is drawn cutting the same branch of the rectangular hyperbola xy = c 2 in Q and R such that PQ = QR. Show that its equation is 9c 2 y + 2b 2 x = 9bc 2.

37.

If a rectangular hyperbola xy = c 2 is cut by a circle passing through its centre O in points A, B, C and D whose parameters are t1 , t2 , t3 and t 4 then show that (t1 + t2 )(t3 + t 4 ) + t1t2 + t3t 4 = 0 and deduce that the product of the perpendicular from O on AB and CD is c 2.

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Chapter

9

Polar Coordinates

9.1  INTRODUCTION A coordinate system represents a point in a plane by an ordered pair of numbers called coordinates. Earlier we used Cartesian coordinates which are directed distances from two perpendicular axes. Now we describe another coordinate system introduced by Newton called polar coordinates which is more convenient for some special purposes.

9.2  DEFINITION OF POLAR COORDINATES We choose a point in the plane and it is called the pole (or origin) and is ­denoted by O. Then we draw a ray (half line) starting at O called polar axis. This is usually drawn horizontally to the right and corresponds to positive x-axis in Cartesian coordinates. P(r, q ) r q O

Polar axis

x

Let P be any point in the plane and r be the distance from O to P. Let q   be the angle (usually measured in radians) between the polar axis and the line OP. Then the point P is represented by the ordered pair (r, q ) and (r, q ) are called the polar coordinates of the point P. We use the convention that an angle is positive if measured in the anti-clockwise direction from the polar axis and negative in the clockwise direction.

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9-2

Analytical Geometry

If P coincides with O then r = q . Then (r, q ) represent the coordinates of the pole for any value of q . Let us now extend the meaning of polar coordinates (r, q ) when r is negative, agreeing that the points (-r, q ) and (r, q ) lie on the same line through O and at the same distance | r | from O but on opposite sides of O. If r > 0, the point (r, q ) lies on the same quadrant as q. If r < 0, then it lies in the quadrant of the opposite side of the pole. We note that the point (r, q ) represents the same point as (r, q  + p ) P(r, q ) x+q

r q x

O P ′(−r, q )

Example 9.2.1 Represent the following polar coordinates in the polar plane: 2p    3p   5p   3,  , ( 4, 5p ) ,  3, −  and  −2  4 3 4

Solution

2p    3p   5p  The coordinates  3,  , ( 4, 5p ) ,  3, −  and  −2  are represented by     4  3 4 points in the following diagram: 5p /4 4 x

3

O

(4, 5p )

x

(3, 5p /4)

3

− 2p / 3

(3, − 2p / 3)

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3p / 4

(− 3, 3p / 4)

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Polar Coordinates

9-3

In Cartesian system of coordinates, every point has only one representation. But in polar coordinates system each point has many representations, for exam 5p  p  −3p   13p   ple, point  3,  is also represented by  3,  ,  3,  ,  −3,  , etc.  4   4   4   4 In general, the point ( r ,q ) is also represented by ( r ,q + 2np ) or ( − r ,q + 2n + 1p ) where n is any integer.

9.3  RELATION BETWEEN CARTESIAN COORDINATES AND POLAR COORDINATES If ( x, y ) is the Cartesian coordinates and ( r ,q ) are the polar coordinates of the x y point P, then cos q = and sin q = . r r Therefore, the transformations from one system to another are given by x = r cos q , y = r sin q . To find r from x and y, we use the relation r 2 = x 2 + y 2 and q is given by

cos q =

x x +y 2

2

y

, sin q =

x + y2 2

y P(r, q ) r y q O

x

x

We have already studied the distance between two points, area of a triangle, equations of a straight line, equations to a circle and equation of conics in Cartesian coordinates system. Let us now derive the results in polar coordinate system.

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9-4

Analytical Geometry

9.4  POLAR EQUATION OF A STRAIGHT LINE The general equation of a straight line in Cartesian coordinates is Ax + By + C = 0 Ax + By + C = 0, where A, B and C are constants. Let (r, q ) be polar coordinates of a point and the x-axis be the initial line. Then for any point (x, y) on the straight line x = r cos q , y = r sin q . Substituting these in the equation of straight line, we get A( r cos q ) + B( r sin q ) + C = 0



(i.e.)  A cos q + B sin q + C / r = 0 This can be written in the form l / r = A cos q + B sin q 



(9.1)

where A, B and l are constants. Therefore, equation (9.1) is the general equation of a straight line in polar coordinates.

9.5  POLAR EQUATION OF A STRAIGHT LINE IN NORMAL FORM Let the origin be the pole and the x-axis be the initial line. Draw ON perpendicular to the straight line. Let ON = p and ∠XON = a. ON In ∆OPN , = cos (q − a ) OP (i.e.)

p = cos (q − a ) r

(i.e.) p = r cos (q − a ) This is the polar equation of the required straight line. y P(r, q ) N p a q O

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x

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9-5

Polar Coordinates

Note 9.5.1:  Polar equation of the straight line perpendicular to A cos q + B sin q = A cos q + B sin q =

l p p l   is of the form A cos  q +  + B sin q +  = k .   r 2 2 r

l r

(i.e.)  − A sin q + B cos q = kl r kl or B cos q − A sin q = , where k is a constant. r



Note 9.5.2:  The polar equation of the straight line parallel to A cos q + B sin q = A cos q + B sin q =

kl l is A cos q + B sin q = , where k is a constant. r r

l r l and A1cosq + B1 sinq = to be perpendicular to each other is AA1 + BB1 = 0. r This result can be easily seen from their cartesian equations.

Note 9.5.3:  The condition for the straight lines A cos q + B sin q =

Note 9.5.4:  If the line is perpendicular to the initial line then a = 0 or p . Therefore, the equation of the straight line is r cos q = p or r cos q = − p .

Note 9.5.5:  If the line is parallel to the initial line then a =

p 3p or . 2 2

In this case the equation of the line is 3p  p   r cos q −  = p or r cos q −  = p (i.e.) r sin q = p or r sin q = − p .   2 2

Example 9.5.6 Find the equation of the straight line joining the two points P ( r1 ,q1 ) and Q( r2 ,q 2 ).

Solution Let R( r ,q ) be any point on the line joining the points P and Q . The area of the triangle formed by the points P ( r1 ,q1 ), Q( r2 ,q 2 ) and ( r3 ,q 3 ) is

1 ∆ = [r1r2 sin(q 2 − q1 ) + r2 r3sin(q 3 − q 2 ) + r3 r1 sin(q1 − q 3 )] 2

Taking r3 = r and q 3 = q , we get

1 ∆ = [r1r2 sin(q 2 − q1 ) + r2 r sin(q − q 2 ) + rr1 sin(q1 − q )] 2

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l is r

9-6

Analytical Geometry

Since the points P , Q and R are collinear, ∆ = 0 .

r1r2 sin (q 2 − q1 ) + rr2 sin (q − q 2 ) + rr1 sin(q1 − q ) = 0

Dividing by r r1 r2 , we get 1 1 1 sin(q 2 − q1 ) = sin(q 2 − q ) + sin(q − q1 ) r r1 r2



This is the equation of the required straight line.

Example 9.5.7 Find the slope of the straight line

Solution

l = cos (q − a ) + e cos q . r

l = cos (q − a ) + e cos q. r l = r (cos q cos a + sin q sin a ) + re cos a

The equation of the straight line is



l = r (e + cos a ) cos q + r sin q sin a (i.e..) l = (e + cos a ) x + y sin a

Therefore, the slope of the straight line is − (e + cos a ) . sin a

Example 9.5.8

l Find the point of intersection of the straight lines = cos (q − q1 ) + e cos q r i and = cos (q − q 2 ) + e cos q . r

Solution The equations of the straight lines are

l = cos (q − q1 ) + e cos q  r i = cos (q − q 2 ) + e cos q  r

(9.2) (9.3)

Solving equations (9.2) and (9.3), we get

cos (q − q1 ) = cos (q − q 2 )

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q − q1 = ± (q − q 2 )

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Polar Coordinates

9-7

q1 + q 2 . 2 Then from the equation of the first straight line, we get Therefore, the only possibility is q =

l q +q  q −q  = cos  2 1  + e cos  1 2   2    r 2



∴r =



l q +q  q −q  cos  2 1  + e cos  1 2   2   2 

Hence, the point of intersection of the two given lines is q +q  . l  , 2 1  q 2 − q1  q1 + q 2  2    cos   + e cos     2   2   



Example 9.5.9

 p  p Find the equation of the line joining the points  2,  and  3,  and  3  6  6 p , . deduce that this line also passes through the point   3 3 − 2 2 

Solution The equation of the line joining the points ( r1 , q1 ) and ( r2 ,q 2 ) is

1 1 1 sin (q 2 − q1 ) = sin (q 2 − q ) + sin (q − q1 ) . r r1 r2

p p Therefore, the equation of the line joining the points  2,  and  3,  is  3  6



1 p p p  1 p  1  sin  −  = sin  − q  + sin q −  .      r 6 3 2 6 3 3

1 p  − sin   =  6 r

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p p   −3 sin q −  + 2 sin q −    6 3 6

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9-8

Analytical Geometry

when q =





1 ∴ = r

p 2

p p 1 3 + 2 cos −3 +2 6 3 = 2 2 = 3 3−2 p 1 6 −6 sin −6 × 6 2

−3 cos



r=

6 3 3−2

 6 p Hence, the point  lies on the straight line. ,  3 3 − 2 2 

Example 9.5.10 Show that the straight lines r (cos q + sin q ) = ±1 and r (cos q − sin q ) = ± 1 ­enclose a square and calculate the length of the sides of this square.

Solution Converting into Cartesian form the four lines are

x+ y = ±1



x− y = ±1

These four lines form a parallelogram and in x + y = ± 1 , x − y = ± 1 the ­adjacent lines are perpendicular and hence ABCD is a rectangle. A

x +y = 1

B x − y = −1

x−y=1 D

x +y = −1

C

Also the distance between AB and CD = 2 . The distance between AD and BC = 2 . Therefore, these four lines form a square.

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Polar Coordinates

9-9

Example 9.5.11

p p   Find the angle between the lines r cos  q −  = p and r cos q −  = p ′.   2 3

Solution The angle between the lines = q 2 − q1 p p − 3 6 p = 6 =

Exercises 1.

Find the angle between the lines



(i) r cos q = p, r sin q = p1



(ii) r cos q = 2, r cos q − p = 3 6

(

)

Ans.: (a)  p    (b)  p 2 6



(

)

6 3 3+2  2. Show that the points 2, p , 3, p and  , p  are collinear. 3 6 25 2  

( )( )

3. Show that the equation of any line parallel to l = cos (q − a ) + e cos q through r  cos a + e  the pole is q = tan −1  .   sin a  4.

Find the equation of the line perpendicular to l = cos(q − a ) + e cos q and r passing through the point ( r1 ,q1 ).



Ans.: r1 sin(q1 − a ) + e sinq1 = sin(q − a ) + e sinq r

9.6  CIRCLE 9.6.1  Polar Equation of a Circle Let O be the pole and OX be the initial line. Let C (c, a ) be the polar coordinates of the centre of the circle. Let P ( r ,q ) be any point on the circle. Then ∠COP = q − a . Let a be the radius of the circle.

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9-10

Analytical Geometry

P(r, q ) a C(c, a) r c a

q

O





x

In ∆ COP , CP 2 = OC 2 + OP 2 − 2OC ⋅ OP cos(q − a ) (i.e.) a 2 = c 2 + r 2 − 2cr cos(q − a ) 

(9.4)

This is the polar equation of the required circle.

Note 9.6.1.1:  If the pole lies on the circumference of the circle then c = a. Then the equation of the circle becomes, a 2 = a 2 + r 2 − 2ar cos (q − a )



(i.e.)

r = 2a cos(q − a )

P(r, q )

r

C(a, a) a

O

x

Note 9.6.1.2:  The equation of the circle r = 2a cos(q − a ) can be written in the form r = A cos q + B sin q where A and Bare constants.

Note 9.6.1.3:  If the pole lies on the circumference of the circle and the initial line passes through the centre of the circle then the equation of the circle becomes, r = 2a cos q since a = 0 .

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9-11

Polar Coordinates

P

O

C

x

Note 9.6.1.4:  Suppose the initial line is a tangent to the circle. Then

c = a cosec a . Therefore, from equation (9.4) the equation of the circle becomes, a 2 = a 2 cos ec2a + r 2 − 2ar cos ec a cos (q − a )

(i.e.) r 2 − 2ra cos ec a cos (q − a ) + a 2 cot 2 a = 0 P(r, q )

r

C(c, a) a

a O

x

Note 9.6.1.5:  Suppose the initial line is a tangent and the pole is at the point of contact. In this case a = 90°. The equation of the circle becomes, r 2 − 2ra sin q = 0 (or ) r = 2a sin q . 9.6.2  Equation of the Chord of the Circle r = 2a cos p on the Line Joining the Points (r1, p1) and (r2, p2). Let PQ be the chord of the circle r = 2a cos q . Q(r2, q 2)

P(r1, q 1) O

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9-12

Analytical Geometry

Let Pand Q be the points ( r1 ,q1 ) and ( r2 ,q 2 ) . Since the points P and Q lie on the circle

r1 = 2a cos q1



r2 = 2a cos q 2

Let the equation of the line PQ be p = r cos (q − a ) 



(9.5)

Since the points P and Q lie on this line

p = 2a cos q1 cos (q1 − a ) 

(9.6)



p = 2a cos q 2 cos (q 2 − a ) 

(9.7)

From equations (9.6) and (9.7), we get

2 cos q1 cos (q1 − a ) = 2 cos q 2 cos (q 2 − a )



(i.e.) cos (2q1 − a ) + cos a = cos (2q 2 − a ) + cos a ∴ 2q1 − a = ± ( 2q 2 − a )



∴ a = q1 + q 2 or q1 = q 2 But q1 ≠ q 2



∴ a = q1+ q 2   

Hence, from equation (9.6), we get p = 2a cos q1 cos q 2 . Hence, from equation (9.5) the equation of the chord is 2a cos q1 cos q 2 = r cos (q1 − q1 + q a cos q1 cos q 2 = r cos (q1 − q1 + q 2 ) .

Note 9.6.2.1:  This chord becomes the tangent at a if q1 = q 2 = a . Therefore, the equation of the tangent at a is 2a cos 2 a = r cos (q − 2a ).

9.6.3  Equation of the Normal at `  on the Circle r = 2` cosp ∠XOP = a        ∴ ∠OPC = a p Since ON is perpendicular to PN , ∠CON = − 2a 2



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Polar Coordinates

9-13

p  ON = a cos  − 2a  2 



= a sin 2a



P ‘a ’ a a

O

a C

x

N

The equation of the normal is p = r cos(q − a ) .

(i.e.)



  p a sin 2a = r cos q − − 2a  2   a sin 2a = r sin (q − 2a )

9.6.4  Equation of the Circle on the Line Joining the Points (a, ` ) and (b, a ) as the ends of a Diameter

AP 2 = a 2 + r 2 − 2ar cos(q − a ) BP 2 = r 2 + b 2 − 2br cos(q − b ) AB 2 = a 2 + b 2 − 2ab cos(a − b ) P(r, q ) (a, a) A

r

(b, b ) B

O

x

Since ∠ APB = 90°

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AB 2 = AP 2 + BP 2

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9-14

Analytical Geometry

a 2 + b 2 − 2ab cos(q − b ) = a 2 + r 2 − 2ar cos(q − a ) + b 2 + r 2 − 2br cos(q − b )

(i.e.) r 2 − r [ a cos(q − a ) + b cos(q − b ) ] + ab cos(q − b ) = 0

Example 9.6.1 Show that the locus of the foot of the perpendicular drawn from the pole to the tangent to the circle r = 2a cos q is r = a(1 + cos q ).

Solution Let P be the point ( r , a ). Draw ON perpendicular to the tangent at P. P N

O

x

The equation of the tangent at P is

r cos(q − 2a ) = 2a cos 2 a

Since ON is the perpendicular distance from O on the line PN, from the normal form of the straight line, we get ON = p = 2a cos 2 a



Let the coordinates of N be ( r1 ,q1 ), then

r1 = 2a cos 2 a and q1 = 2a r1 = a (1 + cos 2a ) r1 = a (1 + cos q1 )

Locus of (r1 ,q1 ) is

r = a (1 + cos q ).

Example 9.6.2 Show that the feet of the perpendiculars from the origin on the sides of the triangle formed by the points with vectorial angles a , b , g

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Polar Coordinates

and which lie on the circle r = 2a cosq 2a cos a cos b cos g = r cos (p − a − b − g ).

9-15

lie on the straight line

Solution The equation of the circle is r = 2a cosq . Let the vectorial angles of P , Q, R be a , b , g respectively. The equations of the chord PQ, QR and RP are 2a cos a cos b = r cos (q − a + b )

2a cos b cos g = r cos (q − b + g ) 2a cos g cos a = r cos (q − g + a )

Let L, M and N be the feet of the perpendiculars from O on the lines PQ, QR and RP Then from the above equations, we infer that the coordinates of L, M and N are (2a cos a cos b , a + b ) (2a cos b cos g , b + g ) (2a cos g cos a , g + a ) These three points satisfy the equation

2a cos a cos b cos g = r cos (q − a − b − g

)

Hence L, M and N lies on the above line.

Example 9.6.3

l Show that the straight line = A cos q + B sin q touches the circle r = 2a cosq r 2 2 2 if a B + 2alA = l .

Solution The equation of the circle is

r = 2a cosq 

(9.8)

The equation of the straight line is

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l = A cos q + B sin q  r

(9.9)

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9-16

Analytical Geometry

Solving these two equations we get their point of intersection. l = A cos q + B sin q 2a cos q



Dividing by cos q , we get

l = A + B tan q 2a cos 2 q

(

)

l 1 + tan 2 q = A + B tan q 2a l tan 2 q + l = 2aA + 2aB tan q l tan 2 q − 2aB tan q + l − 2aA = 0 



(9.10)

If the line (9.9) is a tangent to (9.8) then the two values of tanq of the equation (9.10) are equal. The condition for that is the discriminant is equal to zero. 4 a 2 B 2 − 4l ( l − 2aA) = 0



a 2 B 2 − l 2 + 2aAl = 0

Exercises 1.

Show that r = A cosq + B sinq represents a circle and find the polar coordinates of the centre.

2.

Show that the equation of the circle of radius a which touches the lines q = 0,q = p is r 2 − 2ar (cosq + sinq ) + a 2 = 0. Show that locus of the equation 2 2 r − 2ra cos 2q secq − 2a 2 = 0 consists of a straight line and a circle.

3.

Find the polar equations of circles passing through the points whose polar coordinates are a, p , b, p and touching the straight line q = 0. 2 2 Ans.:  r 2 − r[( a + b)sinq ± 2b cosq ] + c 2 = 0



( )( )

A circle passes through the point ( r ,q ) and touches the initial line at a distance c from the pole. Show that its polar equation is r1 sinq1 ( r 2 − 2ar cosq + c 2 ) = r sinq (r12 − 2cr1 cos 2 2 nq1 ( r − 2ar cosq + c ) = r sinq (r12 − 2cr1 cosq 11 + c). 4.

5.

Show that r 2 − kr cos(q − a ) + kd = 0 represents a system of general circles for different values of k . Find the coordinates of the limiting points and the equation of the common radical axis.

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Polar Coordinates

6.  7.

9-17

( )

Find the equation of the circle whole centre is 4, p and radius is 2. 4 p  Ans.: r 2 − 8 cos  q −  + 12 = 0   4  Find the centre and radius of the circle r 2 − 10 r cosq + 9 = 0.



Ans.:  (5, 0); 4

8.

Prove that the equation to the circle described on the line joining the points 1, p and 2, p as diameter is r 2 − r cos q − p  + 2 cos cos − p + 3 = 0. 3  6  3 6

( ) ( )

9.

Find the condition that the line l = a cosq + b sinq may be a r (i)  tangent



(ii)  a normal to the circle r = 2cosq .

10.

Find the equation of the circle which touches the initial line, the vectorial angle of the centre being a and the radius of the circle a .

11.

A circle passes through the point ( r1 ,q1 ) and touches the initial line at a distance c from the pole. Show that its polar equation is

(

)

(

)

2 2 r 2 − 2cr cosq + c 2 = r1 − 2cr1 cosq + c . r sinq r1 sinq1

9.7  POLAR EQUATION OF A CONIC Earlier we defined parabola, ellipse and hyperbola in terms of focus directrix. Now let us show that it is possible to give a more unified treatment of all these three types of conic using polar coordinates. Furthermore, if we place the focus at the origin then a conic section has simple polar equation. Let S be a fixed point (called the focus) and XM , a fixed straight line (called the directrix) in a plane. Let e be a fixed positive number (called the SP eccentricity). Then the set of all points P in the plane such that = e is PM called a conic section. The conic is (i) an ellipse if e < 1. (ii) a parabola if e = 1 . (iii) a hyperbola if e > 1 .

9.7.1  Polar Equation of a Conic Let S be focus and XM be the directrix. Draw SX perpendicular to the d­ irectrix. Let S be the pole and SX be the initial line. Let P ( r ,q ) be any point

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9-18

Analytical Geometry

on the conic; then SP = r , ∠XSP = q . Draw PM perpendicular to the directrix and PN perpendicular to the initial line. L

y P

M

r q S

N

A

x

L′

Let LSL′ be the double ordinate through the focus (latus rectum). The focus directrix property is SP =e PM





(i.e.) SP = ePM = eNX = e( SX − SN ) l  (i.e.) r = e  − r cos q  e 

r = l − er cos q r (1 + e cos q ) = l l or = 1 + e cos q r This is the required polar equation of the conic.

Note 9.7.1.1:  If the axis of the conic is inclined at an angle a to the initial line then the polar equation of conic is

l = 1 + e cos(q − a ). r P (r, q )

S

q−a a

x

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Polar Coordinates

9-19

l = 1 + e cos q . r cosq is a periodic function of period 2p . Therefore, to trace the conic it is enough if we ­consider the variation of q from −p to p . Since cos( −q ) = cos q the curve is symmetrical about the initial line. Hence it is enough if we study the variation of q from 0 to p . Let us discuss the various cases for different values of q . To trace the conic,

Case 1: Let e = 0. In this case, the conic becomes r = l which is a circle of radius l with its centre at the pole. Case 2: Let e = 1. In this case, the equation of the conic becomes, l l = 1 + cos q or r = . When q varies from 0 to p , r 1 + cos q

1+ cosq varies from 2 to 0. 1 1 varies from to ∞ . and 1+ cosq 2 The conic in this case is a parabola and is shown below.

S

l 1 + e cosq As q varies from 0 to p , 1+ e cosq decreases from 1+ e to 1 − e. r increases from l to l 1+ e 1− e The curve is clearly closed and is symmetrical about the initial line. The conic is an ellipse. Case 3: Let e < 1 . r =

−

l ,0 1+e

A′

S

A

x l ,0 1+e

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9-20

Analytical Geometry

Case 4: Let e > 1. r =

l . 1 + e cos q

As q varies from 0 to p , 1+ e cosq decreases from (1+ e) to 1  2 and hence r increases from l to l . 1+ e As q varies from p to p , 1+ e cosq decreases from 1 to (1− e) . 2 p Therefore, there exists an angle a such that < a < p at which 2 1 1 + e cos q > 0. (i.e.) cos a > − . e p to a , r increases from 1 to ∞ . As q Hence, as q varies from 2 varies from a to p , 1+ e cosq remains negative and varies from 0 to (1 − e). 1 . r varies from − ∞ to 1− e

a S

O

A′

A

1 ,0 1+e



1 ,0 1−e

x

The conic is shown above and is a hyperbola.

9.7.2  Equation to the Directrix Corresponding to the Pole Let Q be any point on the directrix. Let its coordinates be ( r ,q ). Then SX = r cosq l l = r cos q or = e cos q . Since this is true for all points ( r ,q ) on the directrix, e r l the polar equation of the directrix is = e cosq . r (r, q ) q

X

x

S

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9-21

Polar Coordinates

Note 9.7.2.1:  The equation of the directrix of the conic

l = 1 + e cos(q − a ) r

l = e cos(q − a ). r The polar equation of the conic for different form of directrix is given below. is

y

x Directrix

S

x

S Directrix

l = 1 − e cosq r

l = 1 + e cosq r y

Directrix

x

S Directrix

l = 1 − e sinq r

l = 1 + e sinq r

Note 9.7.2.2:  The above conic is an ellipse if e < 1, parabola if e = 1 and hyperbola if e > 1.

9.7.3  Equation to the Directrix Corresponding to Focus Other than the Pole Let ( r ,q ) be the coordinates of a point on the directrix X ′M ′. M′

P

Q(r, q ) 180 − q

X′

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M

q S

x

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9-22

Analytical Geometry

Then But



SX ′ = SQ cos(180° − q ) = − SQ cos q = − r cos q SX ' = XX ' − SX 2a l 2a − l = − = e e e b 2 a 2 (1 − e 2 ) = = a(1 − e 2 ) l= a a 2l l − r cos q = − 2 e(1 − e ) e l 2  − 1 2  e 1 − e  l  2 − 1 + e2  =   e  1 − e2  =

=



l 1 + e 2    e 1 − e 2 

l  e 2 − 1 = e cosq r  e 2 + 1 This is the required equation of the other directrix.

9.7.4  Equation of Chord Joining the Points whose Vectorial Angles are `  - a  and `  + a  on the Conic Let the equation of the conic be l = 1 + e cosq . r l Let the equation of the chord PQ be = A cos q + B cos(q − a ). r Q P

S

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Polar Coordinates

9-23

This chord passes through the point ( SP , a − b ) and ( SQ, a + b ) . l = A cos(a − b ) + B cos b  SP  l = A cos(a + b ) + B cos b SQ



Also these two points lie on the conic



(9.12)

l = 1 + e cos q , r

l = 1+ e cos (a − b )  SP l = 1 + e cos (a + b ) SQ



(9.11)

(9.13) (9.14)

From equations (9.11) and (9.13), we get A cos (a − b ) + B cos b = 1 + e cos (a − b ) 



(9.15)

From equation (9.12) and (9.14), we get A cos (a + b ) + B cos b = 1 + e cos (a + b ) 



(9.16)

Subtracting, we get A[cos(a − b ) − cos(a + b )] = e[cos(a − b ) − cos(a + b )] A=e From equation (9.15), we get e cos(a − b ) + B cos b = 1 + e cos(a − b )



B cos b = 1 (i.e.) B = sec b

The equation of the chord PQ is l = e cos q + sec b cos(q − a ). r

9.7.5  Tangent at the Point whose Vectorial Angle is `  on the Conic l = 1 + e cosp r The equation of the chord joining the points with vectorial angles l a − b and a + b is = e cos q + sec b cos(q − a ). r This chord becomes the tangent at a if b = 0. l The equation of tangent at a is = 1 + e cos q + cos(q − a ). r

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9-24

Analytical Geometry

9.7.6  Equation of Normal at the Point whose Vectorial Angle is `   on the Conic The equation of the conic is

l = 1 + e cosq r

l l The equation of tangent at a on the conic = 1 + e cosq is = e cosq + r r cos(q − a ). P

S

k p  The equation of the line perpendicular to this tangent is = e cos q +  +  r 2 p   cos q + − a  .   2

(i.e.)

k = −e sin q − sin(q − a ) r

If this perpendicular line is normal at P, then it passes through the point ( SP , a ).

k = −e sin a or k = − SP ⋅ e sin a  SP

Since the point ( SP , a ) also lies on the conic



l = 1 + e cos q , we have r

l = 1 + e cos a SP l SP = 1 + e cos a

From equation (9.17), we get k =

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 24

(9.17)

le sin a . 1 + e cos a

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Polar Coordinates

9-25

Hence, the equation of the normal at a is



−

1 le sin a = −e sin q − sin(q − a ) r l + e cos a 1 le sin a = e sin q + sin (q − a ) r l + e cos a

(i.e.)

9.7.7  Asymptotes of the Conic is The equation of the conic is

l r

= 1 + e cos p (e > 1)

l = 1 + e cosq  r



(9.18)

The equation of the tangent at a is l = e cos q + cos(q − a )  r



(9.19)

This tangent becomes an asymptote if the point of contact is at infinity, that is, the polar coordinates of the point of contact are (∞ , a ). Since this point has to satisfy the equation of the conic (9.18) we have from equation (9.18), 0 = 1 + e cosa 



or cosa = −

(9.20)

1 e

l = (e + cos a ) cos q + sin a sin q . r 1 1 Substituting cos a = − and sin a = ± 1 − 2 , we get the equation of the e e l  1 1 asymptotes as =  e −  cos q ± 1 − 2 sin q .   r e e The equation (9.19) can be written as



(i.e.)



∴

l e2 − 1 e2 − 1 = cos q ± sin q r e e  l e2 − 1  1 = sin q  cos q ± 2 r e  e −1 

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9-26

Analytical Geometry

9.7.8  Equation of Chord of Contact of Tangents from (r1,p1) to the Conic l = 1 + e cosq r



Let QR be the chord of contact of tangents from P ( r1 ,q1 ). Let vectorial angles of Q and R be a − b and a + b . The equation of the chord QR is

l = e cos q + sec a cos(q − b )  r

(9.21)

The equations of tangents at Q and R are

l = e cos q + cos(q − a − b )  r

(9.22)



l = e cos q + cos(q − a + b )  r

(9.23)

These two tangents intersect at ( r1 ,q1 ) . l = e cos q1 + cos(q1 − a − b )  r1

(9.24)

l = e cos q1 + cos(q1 − a − b )  r1 From the above two equations, we get

(9.25)





cos(q1 − (a − b ) = cos(q1 − a + b ) q1 − a − b = ±(q1 − a + b ) q1 − a − b = q1 − a + b ⇒ b > 0 which is inadmissible q1 − a − b = −q1 + a + b ⇒ q1 = a Substituting this in equation (9.24), we get

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 26

l = e cos q1 + cos b  r1

(9.26)

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Polar Coordinates

9-27

Substituting equation (9.26) in (9.21), we get



l 1 − e cos q = ⋅ cos(q − q1 ) r l   r − e cos q1  1  l  l (i.e.)  − e cos q   − e cos q1  = cos(q − q1 ) r   r1 

This is the equation of the chord of contact.

9.7.9  Equation of the Polar of any Point (r1,p1) with Respect to the l Conic = 1 + e cosp r The polar of a point with respect to a conic is defined as the locus of the point of intersection of tangents at the extremities of a variable chord passing through the point P ( r1 ,q1 ). Q

P(r1, q1) T (R, f) R

Let the tangents at Q and R intersect T . Since QR is the chord of contact of tangents from T ( R, f ), its equation is

l  l   − e cos q   − e cos f  = cos(q − f )  r R

(9.27)

Since this passes through the point P ( r1 ,q1 ) we have

l  l    r − e cos q1   R − e cos f  = cos(q1 − f )

(9.28)

1

Now the locus of the point T ( R, f ) is polar of the ( r1 ,q1 ).

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9-28

Analytical Geometry

The polar of ( r1 ,q1 ) from equation (9.28) is  l  l  − e cos q   − e cos q1  = cos(q − q1 ). r  r1 



Example 9.7.1 l Find the condition that the straight line = A cos q + B sin q may be a tanr l gent to the conic = 1 + e cos q . r

Solution Let the line

l = A cos q + B sin q touches the conic at the point ( r , a ). r P(r, a)

S

Then the equation of tangent at ( r, a ) is l = e cos q + cos(q − a )  r



l = (e + cos a ) cos q + sin a sin q  r However, the equation of tangent is given as l = A cos q + B sin q  r Equations (9.30) and (9.31) represent the same line. Identifying equations (9.30) and (9.31), we get

(i.e.)



A = e + cos a (or) A − e = cos a



B = sina

(9.29) (9.30)

(9.31)

Squaring and adding, we get ( A − e)2 + B 2 = 1 This is the required condition.

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Polar Coordinates

9-29

Example 9.7.2 Show that in a conic, semi latus rectum is the harmonic mean between the segments of a focal chord.

P

S

Q

Solution

l = 1 + e cos q . Let P and Q have the r polar coordinates ( SP , a ) and ( SQ, a + p ). l Since P and Q lie on the conic = 1 + e cosq . r We have   l = 1 + e cosa  (9.32) SP l = 1 − e cosa  (9.33) SQ Let PQ be a focal chord of the conic

Adding equations (9.32) and (9.33) l l + =2 SP SQ

or

1 1 2 + = SP SQ l

SP , l , SQ are in HP (i.e.) l is the HM between SP and SQ.

Example 9.7.3 Show that in any conic the sum of the reciprocals of two perpendicular focal chords is a constant.

Solution Let PP′ and QQ′ be perpendicular focal chords of the conic

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9-30

Analytical Geometry

l = 1 + e cosq  r

Let

(9.34)

P be the point ( SP , a ). The vectorial angles of Q, P ′, Q ′ are

p 3p a + , a + p and a + . 2 2 Since the points P, P ′, Q, Q ′ lie on the conic, Q S

P′

P

Q′

we have

l = 1 + e cos a SP

∴ SP =

1 1 + e cos a



l = 1 − e cos a SP ′

∴ SP' =

l 1 − e cos a



l = 1 − e sin a SQ

∴ SQ =

l 1 − e sin a



l = 1 + e sin a SQ ′



PP ′ = SP + SP ′ =



QQ ′ = SQ ′ + SQ ′ =





∴ SQ ′ =

l 1 + e sin a

2l l l + = 2 1 + e cos a 1 − e cos a 1 − e cos 2 a 2l l l + = 2 1 − e sin a 1 + e sin a 1 − e sin 2 a

1 1 1 − e 2 cos 2 a 1 − e 2 sin 2 a + = + PP ′ QQ ′ 2l 2l =

2 − e2 = constant. 2l

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Polar Coordinates

9-31

Example 9.7.4 If a chord PQ of a conic whose eccentricity e and the semi ­latus rectum l subtends a right angle at the focus SP then prove that 2 2 e2  1 1  1 1 − + − = .   SP l   SQ l  l2

Solution

l = 1 + e cos q . Let the vectorial angle of P r p be a . The vectorial angle of Q is a + . 2 Let the equation of the conic be

Q

P

x

S

Since P and Q lie on the conic,



l l = 1 + e cos a and = 1 − e sin a SP SQ 1 1 + e cos a = SP l 1 1 e cos a − = SP l l

Similarly, 1 1 e sin a − =− SQ l l

Squaring and adding, we get

2



2

e2  1 1  1 1 −  + −  = 2.  SP l   SQ l  l

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9-32

Analytical Geometry

Example 9.7.5 Let PSQ and PS ′R be two chords of an ellipse through the foci S and S ′. Show that SP + S ′P is a constant. SQ S ′R

Solution Let the vectorial angle of P be a . Then the vectorial angle of Q is a + p . l Since P and Q lie on the conic = 1 + e cos q r

l = 1 + e cos a  SP

(9.35)



l = 1 − e cos a  SQ

(9.36)



1 1 + e cos a = SP l P S′ S R′ Q

1 1 − e cos a = SQ l





1 1 2 l l + = or + =2 SP SQ l SP SQ



(9.37)

Similarly, considering the other focal chord PS ′R

l l + =2 S ′P S ′Q

Multiply equation (9.37) by SP , we get l SP 2SP = −1 SQ l

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 32



(9.38)



(9.39)

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Polar Coordinates

9-33

Similarly from equation (9.38), we get S ′P 2S ′P (9.40) = −1  S ′R l Adding equations (9.39) and (9.40), we get SP + S ′P = 2 ( SP + S ′P ) − 2 SQ S ′R l





=

2 4a ( 2a) − 2 = − 2 = a constant l l

Example 9.7.6 Prove that the perpendicular focal chords of a rectangular hyperbola are equal.

Solution Let PSP ′ and QSQ ′ and be two perpendicular focal chords of a rectangular hyperbola. Then the vectorial angles of P and P ′ are a . y

Q

P′ P S′

S

x

Q′



l = 1 + e cos a SP

l = e cos a −1 SP ′ Since P ′ lies on the other branch of the hyperbola, the polar equation of the l conic is = −1 + e cos a . r



SP =

l 1 + e cos a



SP ′ =

l e cos a − 1

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9-34

Analytical Geometry



1 l − e cos a − 1 e cos a + 1 l[e cos a + 1 − e cos a + 1] 2l = = 2 e 2 coos 2 a − 1 e cos 2 a − 1

PP ′ = SP ′ − SP =



=

2l (since in a RH e = 2 )  cos 2a

(9.41)

Similarly, l 1 + e sin a l SQ ′ = 1 − e sin a SQ =



2l 1 − e 2 sin 2 a = 2l  cos 2a

SQ + SQ ′ =

(9.42)

From equations (9.41) and (9.42), we get PP ′ = QQ ′. That is, in a RH , perpendicular focal chords are of equal length.

Example 9.7.7 The tangents to a conic at P and Q meet at T . Show that if S is a focus then ST bisects ∠PSQ .

Solution Let the equation of the conic be l = 1 + e cos q . The equation of the tangent r at P with vectorial angle a is l = e cos q + cos (q − a ). r Q

T

S P

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Polar Coordinates

9-35

The equation of the tangent at Q with vectorial angle b is l = e cos q + r cos(q - b ). At the point of intersection of these two tangents,

cos(q − a ) = cos(q − b )



∴ (q − a ) = ±(q − b )



(i.e.) a = b or q =



a = b is inadmissible ∴q =

a +b 2

a +b 2 (i.e.) ST bisects ∠PSQ.



Example 9.7.8 If the tangents at the extremities of a focal chord through the focus S of the conic l = 1 + e cos q meet the axis through S in T and T ′ show that r 1 + 1 = 2e . ST ST ′ l

Solution Let PSQ be a focal chord. Let the vectorial angles of P and Q be a and a +p. Then the equations of tangents at P and Q are

l = e cos q + cos(q − a )  r

(9.43)



l = e cos q + cos(q − a +p )  r

(9.44)

(i.e.) l = e cos q − cos (q − a )  r When the tangents meet the axis, at those points q = 0.

(9.45)



l = e + cos a ST l = e − cos a ST ′



Adding

l + l = 2e ST ′ ST ′

or

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 35

l + 1 = 2e ST ST ′ l

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9-36

Analytical Geometry

Example 9.7.9 If a chord of a conic l = 1 + e cosq subtends an angle 2a at the focus then r show that the locus of the point where it meets the internal bisector of the angle is l = seca + ecosq . r

Solution Let PQ be a chord of the conic l = 1 + e cos q subtending an angle 2a at r the focus. Let the internal bisector of PSQ meets PQ at T . Let the vectorial angles of P and Q be b − a and b + a . Let the polar coordinates of T be ( r1 , b ). Q T S

P x

The equation of the chord PQ is l = e cos q + sec a cos(q − b ) r This passes through the point T ( r1 , b ).





∴ l = e cos b + sec a r1

The locus of ( r1 , b ) is l = seca + e cos q. r

Example 9.7.10 The tangents at two point P and Q of the conic meet in T and PQ subtends a constant angle 2a at the focus. Show that 1 + 1 − 2 cos a is a SP SQ ST constant.

Solution Let the equation of the conic be l = 1 + e cos q . Let the vectorial angles of r P and Q be b − a and b + a .

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9-37

Polar Coordinates

Q

T

P S

Since the points P and Q lie on the conic,

l = 1+ e cos(b − a ) SP



1 + e cos(b − a )  ∴ 1 = SP l



l = 1 + e cos(b +a ) SQ

1 + e cos(b +a )  ∴ 1 = SQ l Also the equation of chord PQ is



(9.46)

(9.47)

l = e cos q + sec a cos(q − b ) r



l − e cos q = sec a cos (q − b )  r PQ is also the polar of the point T and so its equation is



( rl − e cos q ) ( STl − e cos b ) = cos(q − b ) 



(9.48)

(9.49)

Identifying equations (9.48) and (9.49), we get 1 = cosa + e cos b ST l





(9.50)

From equations (9.46) and (9.47) and (9.50), we get 1 + 1 − 2 cos a SP SQ ST



=

1 + e cos( b − a ) 1 + e cos ( b + a ) 2 cos a (cos a + e cos b ) + − l l l

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9-38



Analytical Geometry

=

2 + e [e cos ( b − a ) + cos ( b + a )] − 2 cos 2 a − 2e cos a cos b l

=

2(1 − cos 2 a ) 2 sin 2 a which is a constant. = l l

Example 9.7.11 If a focal chord of an ellipse makes an angle a with the major axis then show  2e sin a  . that the angle between the tangents at its extremities is tan − 1   1 − e 2  P (SP, a)

S Q

Solution Let the equation of the conic be l = 1 + e cos q  r The equation of the tangent at P is



(9.51)



l = e cos q + cos(q − a )  r

(9.52)



(i.e.) l = (e + cos a ) cos q + sin a sin q  r

(9.53)

The equation of the tangent at Q is

l = e cos q + cos(q − a + p )  r

(9.54)



l = e cos q − cos(q − a )  r

(9.55)



l = (e − cos a ) cos q − sin a sin q  r

(9.56)

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9-39

Polar Coordinates

Transforming into cartesian coordinates by taking x = r cosq , y = sin q Equations (9.53) and (9.54) becomes, l = (e + cos a ) x + y sin a l = (e − cos a ) x − y sin a



The slopes of the tangents are

m1 =

−(e + cos a ) , m2 = e − cos a sin a sin a

If q is the angle between the tangents then  m − m2  tanq = ±  1  1 + m1m2 



−(e + cos a ) e − cos a − sin a sin a =± e a e cos cos a + − 1− . sin a sin a



(e + cos a + e − cos a ) sin a sin 2 a − (e 2 − cos 2 a ) = ± 2e sin2a 1− e =±



The acute angle between the tangents is given by

(

q = tan −1 2e sin2a 1− e



)

Example 9.7.12 A focal chord SP of an ellipse is inclined at an angle a to the major axis. Prove that the perpendicular from the focus on the tangent at P makes with the axis an angle tan −1 sin a . e + cos a

(

)

Solution Let the equation of the conic be

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 39

l = 1 + e cosq  r

(9.57)

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9-40

Analytical Geometry

L P S

x

The equation of tangent at P is l = 1 + e cos q + cos(q − a )  r The equation of the perpendicular line to the tangent at P is



(

) (

k = e cos q + p + cos q + p − a 2 2 r



(9.58)

)

(i.e.) k = −e sin q − sin(q − a ) r If the perpendicular passes through the focus then k = 0



−e sin q − sin(q − a ) = 0

(i.e.) e sin q + sin q cos a − cos q sin a = 0 sin a e + cos a



tan q =



or q = tan −1

( e +sincosa a )

Example 9.7.13 (i) If A circle passing through the focus of a conic whose latus rectum is 2l meets the conic in four points whose distances from the focus are r1, r2 , r3, r4 then prove that 1 + 1 + 1 + 1 = 2 . r1 r2 r3 r4 l (ii) A circle of given radius passing through the focus S of a given conic intersects in A, B, C and D. Show that SA ⋅ SB ⋅ SC ⋅ SD is a constant.

Solution Let the equations of the conic be

l = 1 + e cosq  r

(9.59)

Let a be the radius of the circle and a be the angle the diameter makes with the initial line.

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Polar Coordinates

9-41

Then the equation of the circle is r = 2a cos(q − a ) 



(9.60)

Eliminating q between equations (9.59) and (9.60) we get an equation whose roots are the distances of the point of intersection from the focus. Form equation (9.60), we get r = 2a (cos q cos a + sin q sin a ). From equation (9.59), we get cosq = l − r . re Estimating q , we get

( )  (i.e.) r = cos a l − r + sina 1 − ( l − r ) re 2a re ( 2ra − 1re− r cosa ) = 1 − ( l re− r )  sin a  r = 2a cos a l − r + sin a 1 − l − r re re 



2

2

2



2

2



2 2 r 2 − l − r cos a + (l − r ) cos 2 a = sin 2 a − (l − r ) sin 2 a 2 2 2 2 2 ae e r e r 4a



2 r 2 − 1 − r cos a + (l − r ) − sin 2 a = 0 2 2 2 ae e r 4a

e 2 r 4 + e r 3 cos a +  − el + 1 − e 2 sin 2 a  r 2 − 2lr + l 2 = 0  4a2 a  a  Dividing by r 4 and rewriting the equation in power of 1 , we get r 2 l − 2l + 1 − e 2 sin 2 a − el 1 − e cos a + e 2 = 0  a r2 ar 4a2 r4 r3

(

)

(9.61)

()

(9.62)

If r1, r2 , r3, r4 are the distances of the points of intersection from the focus then 1 , 1 , 1 , 1 are the roots of the above equation. r1 r2 r3 r4 1 + 1 + 1 + 1 = 2l = 2 r1 r2 r3 r4 l 2 l Form equation (9.61), we get 2 2 r1r2 r3 r4 = 4 a 2l = a constant. e



(i.e.) SA ⋅ SB ⋅ SC ⋅ SD is a constant.

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9-42

Analytical Geometry

Example 9.7.14 If a chord of the conic l = 1 + e cosq subtends a constant angle 2b at the pole r then show that the locus of the foot of the perpendicular from the pole to the chord (e 2 − sec2 b )r 2 − elr cosq + l 2 = 0.

Solution Let the vectorial angles of P and Q be a − b and a + b . Q(a + b ) T P(a − b ) S

The equation of the chord PQ is

l = e cos q + sec b cos(q − a )  r

The equation of the line perpendicular to this chord is k = −e sinq − secb sin(q − a )  r

(9.63)

(9.64)

This line passes through the focus S and so k = 0.

0 = −e sin q − sec b sin(q − a ) 

(9.65)

From equation (9.63), we get l − e cos q = sec b cos(q − a )  r From equation (9.65), we get





−e cos q = sec b sin(q − a ) 

(9.66)

(9.67)

Squaring and adding (9.66) and (9.67), we get

( rl − e cosq ) + e sin q = sec b



l 2 − 2erl cos q + e 2 r 2 cos 2 q + e 2 r 2 sin 2 q − r 2 sec2 b = 0



r 2 (e 2 − sec2 b ) − 2elr cos q + l 2 = 0

2

2

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 42

2

2

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Polar Coordinates

9-43

Example 9.7.15 A variable chord of conic subtends a constant angle 2b at the focus of the conic l = 1 + e cosq . Show that the chord touches another conic having the r same focus and directrix. Show also that the locus of poles of such chords of the conic is also a similar conic.

Solution Let PQ be a chord of the conic l = 1 + e cosq subtending a constant angle r 2a at the focus. Q

T P

S

x

Let T ( r1 ,q ) be the point of intersection of tangents at P and Q. Then PQ is the polar of T and T is the pole of PQ. Let the vectorial angles of P and Q be a − b and a + b . Then the equation of chord PQ is l = e cos q + sec b cos(q − a ) r Dividing by sec b , l cos b = e(sec b ) cos q + cos(q − a ) r i.e. L = E cos q + cos(q − a ) r where L = e cos b and E = e secq . This line is a tangent to the conic C ′ . L = 1 + E cosq r This equation has the same focus as l = 1 + e cos q . Hence, the conic C ′ has r the same focus and the same initial line as C. For the given conic SX = l . e l cos b For the conic C ′, SX ′ = L = = l e e cos b e ∴ SX = SX ′. Hence X ′ coincides with X .

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9-44

Analytical Geometry

Hence, both the conics have the same focus and the same directrix. The equation of tangents at P and Q are l = e cos q + cos(q − a + b ) and r l = e cos q + cos(q − a − b ). r These two tangents intersect at T ( r1 ,q1 ) l = e cos q + cos(q − a − b )  (9.68) 1 1 r1

l = e cos q + cos(q − a + b )  1 1 r1

(9.69)

From equations (9.68) and (9.69), we get e cos q1 + cos(q1 − a + b ) = e cos q1 + cos(q1 − a − b ) cos(q1 − a + b ) = cos(q1 − a − b ) q1 − a + b = ±(q1 − a − b ) b = 0 or q1 = a But b ≠ 0 Substituting q1 = a in equation (9.68), we get l = e cos q + cos a 1 r1 The locus of ( r1 ,q1 ) is l = cos q + e cos q . r (i.e.) l sec a = 1 + e sec a cos q r L ′ = 1 + E ′ cos q where L′ = l secs b , E ′ = e seca r The locus of poles is the conic having the same focus and same directrix as the given conic.

Example 9.7.16 Show that the locus of the point of intersection of tangents at the extremities of a variable focal chord is the corresponding directrix.

Solution Let the equation of the conic be l = 1 + e cosq . r l The equation of tangent at a is = 1 + e cos q + cos(q − a ) . r The equation of tangent at a + p is l = e cos q + cos(q − a + p ) . r l ∴ = e cos q − cos(q − a ) r

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Polar Coordinates

9-45

Let ( r1 ,q1 ) be the point of intersection of these two tangents. Then, l = e cos q1 + cos(q1 − a ) r1 l = e cos q − cos(q − a ) 1 1 r1 Adding these two equations, we get



2l = 2e cos q or l = e cos q 1 1 r1 r1

Therefore, the locus is the corresponding directrix l = e cos q . r

Example 9.7.17 Show that the locus of the point of intersection of perpendicular tangents to a conic is a circle or a straight line.

Solution Let the equation of the conic be l = 1 + e cosq  r



(9.70)

Let P and Q be the points on the conic whose vectorial angles are a and  b . The equations of tangents at P and Q are P T Q‘b’



l = e cos q + cos(q − a )  r l = e cos q + cos(q − b )  r

(9.71) (9.72)

Let ( r1 ,q1 ) be the point of intersection of tangents at P and Q. Then,

l = e cos q + cos(q − a )  1 1 r1

M09_ANALYTICALGEOMETRY_ISBN_CH09.indd 45

(9.73)

4/22/2013 9:27:57 PM

9-46

Analytical Geometry

l = e cos q + cos(q − b )  1 1 r1



(9.74)

From equations (9.73) and (9.74), we get cos (q1 − a ) = (q1 − b )



q1 − a = ±(q1 − b ) a +b a = b or q1 = 2 But a = b is not possible. a +b ∴q 1 = 2 a − b l − e cos q Form equation (9.73), we get cos = r1 2 Expanding equations (9.70) and (9.71), we get l = (e + cos a ) cos q + sin a sin q r l = (e + cos b ) cos q + sin b sin q r Since these two lines are perpendicular, we have



(e + cos a )(e + cos b ) + sin a sin b = 0



(i.e.) e 2 + e(cos a + cos b ) + cos(a − b ) = 0



a +b a −b a −b  e 2 + e  2 cos cos + 2 cos 2 −1 = 0  2 2  2

Substituting for a + b and a − b , we get 2 2 2



    e 2 + 2e cos q1  l − e cos q1  + 2  l − e cos q1  − 1 = 0  r1   r1 



(e 2 − 1) − 2el cos q1 + 2l2 = 0 r1 r1 2

(1 − e 2 )r12 + 2elr1 cos q1 − 2l 2 = 0 Therefore, the locus of ( r1 ,q1 ) is (1 − e 2 )r 2 + 2elr cos q − 2l 2 = 0 .

Example 9.7.18 Prove that points on the conic l = 1 + e cosq whose vectorial angles are a and r b b , respectively, will be the extremities of a diameter if e + 1 = tan a tan . e −1 2 2

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Polar Coordinates

9-47

Solution The equation of the conic is l = 1 + e cos q . r Let a , b be the extremities of a diameter of the conic l = 1 + e cosq . Then r the tangents at a and b are parallel and hence their slopes are equal. The equation of tangent at a is l = e cos q + cos(q − a ) r l = (e + cos a ) cos q + sin a sin q r l = (e + cos a ) x + y sin a (in Cartesian form)

(

)

The slope of this tangent is − e + cos a . sin a Since tangents at a and b are parallel,  e + cos b  − e + cos a = −  sin a  sin b  (e + cos a ) sin b = (e + cos b ) sin a

(

)

e(sin b − sin a ) = sin(a − b ) b +a b −a  a −b a −b  = 2 sin e  2 cos sin cos   2 2 2 2 





b −a 2 b +a − cos 2 b b −a cos − cos e +1 = 2 b −a b e −1 cos + cos 2 b a 2 sin sin 2 2 = b a 2 cos cos 2 2 b = tan a tan 2 2 e= 1

cos

+a 2 +a 2

Example 9.7.19 If a normal is drawn at one extremity of the latus rectum, prove that the distance from the focus of the other point in which it meets the conic is (1 + 3e 2 + e 4 )l . 1 + e2 − e4

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9-48

Analytical Geometry

Solution The equation of the conic is

l = 1 + e cosq  r

(9.75)

L

S

Q

x

L'

( )

The equation of the normal at L l, p is 2 le = e sin q - cos q  (9.76) r Solving equations (9.75) and (9.76), we get their point of intersection e sin q − cos q = e(1 + e cos q )



e sin q = e + (1 + e 2 ) cos q Squaring e 2 sin 2 q = [e + (1 + e 2 ) cos q ]2 e 2 (1 − cos 2 q ) = e 2 + (1 + e 2 )2 cos 2 q + 2e(1 + e 2 ) cos q (e 4 + 3e 2 + 1) cos 2 q + 2e(1 + e 2 ) cos q = 0 cos q = 0 or cosq = −

2e(1 + e 2 ) e 4 + 3e 2 + 1

If cos q = 0 then q = p . This corresponds to the point L. 2 2e(1 + e 2 ) At the other end of the normal cosq = − 4 e + 3e 2 + 1 Substituting in equation (9.75) we get,

2 2 l = 1 − 2e (1 + e ) 2 r 1 + 3e + e 4 l (1 + 3e 2 + e 4 ) r= = SQ 1 + e2 − e4

Example 9.7.20 If the tangents at the points P and Q on a conic intersect in T and the chord PQ meets the directrix at R then prove that the angle TSR is a right angle.

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Polar Coordinates

9-49

Solution Let the vectorial angles of P and Q be a and b , respectively. Let the tangents at P and Q meet at T ( r1 ,q1 ) . Q

T(r , 1 q ) 1

P

The equation of tangents at P and Q are l = e cos q + cos(q − b ) . r Since these tangents meet at ( r1 ,q1 ) , we have l = e cos q + cos(q − b ) . 1 1 r

l = e cos q + cos(q − a ) and r l = e cos q + cos(q − a ) and 1 1 r

∴ cos(q1 − a ) = cos(q 2 − b )

a +b 2 Let q be the vectorial angle of R . The equation of chord PQ is l = sec b − a cos q − a +b  + e cos q .  2 2  r The equation of the directrix l = e cosq . r b −a a +b   Subtracting, we get sec cos q − =0 2 2   q1 − a = ±(q 2 − b ) which impliesq1 =



a +b   cos q − =0  2  a +b p q− = 2 2 q − q1 = p 2 ∠TSR = 90°

Example 9.7.21 A chord PQ of a conic subtends a constant 2g at the focus S and tangents 2 at P and Q meet in T . Prove that 1 + 1 − 2 cos g = 2 sin g . SP SQ ST l

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9-50

Analytical Geometry

Solution Let the equation of the conic be l = 1 + e cos q . r Q q

T(r, q ) P

x

Let the vectorial angles of P and Q be a and b , respectively. l = 1 + cos b . Since these points lie on the conic, l = 1 + e cosa and SQ SP If the tangents at P and Q intersect at ( r1 ,q1 ) then q = a + b . 1 2 l = cos b − a + e cos a +b 2 2 r1



Since PQ subtends an angle 2g at S, a − b = 2g





l = cos g + e cos a + b ST 2 1 + 1 − 2 cos g SP SQ ST  = 1 (1 + e cos a ) + (1 + e cos b ) − 2 cos g l

a + b    cos g + e cos 2   

 a + b   = 1  2 + e  (cos a + cos b ) − 2 cos 2 g − 2e cos g cos 2   l   a +b a +b a −b  − e2 cos g sin = 1 2(1 − cos 2 g ) + e  2 cos cos 2 2  2   l a +b a +b  = 1 2 sin 2 g + e2 cos cos g − e.2 cos g .cos l 2 2  = 2 sin 2 g l

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Polar Coordinates

9-51

Exercises 1. 2. 3. 4.

If PSP ′ and QSQ ′ are two focal chords of a conic cutting each other at right 1 + 1 angles then prove that = a constant. SP .SP ′ SQ.SQ ′ If two conics have a common focus then show that two of their common chords pass through the point of intersection of their directrices. Show that 1 = 1 + e cosq and l = −1 + e cosq represent the same conic. r r In a parabola with focus S, the tangents at any points P and Q on it meet at T . Prove that



(i) SP ⋅ SQ = ST 2 (ii) The triangles SPT and SQT are similar.

5.

If S be the focus, P and Q be two points on a conic such that the angle PSQ is constant, prove that the locus of the point of intersection of the tangents at P and Q is a conic section whose focus is S . If the circle r + 2a cosq = 0 cuts the conic 1 = 1 + e cos(q − a ) in four real r points find the equation in r which determines the distances of these points from the pole. Also, show that if their algebraic sum equals 2a and the eccentricity of the conic is 2cosa . l l Prove that the two conics 1 = 1 + e1 cosq and 2 = 1 + e cos(q − a ) touch each r r other if l12 (1 − e22 ) + l22 (1 − e12 ) = 2l1l2 (1 − e1e2 cos a ). P , Q and R are three points on the conic l = 1 + e cosq . Tangents at Q meets r SP and SR in M and N so that SM = AN = l where S is the focus. Prove that the chord PQ touches the conic. Prove that the portion of the tangent intercepted between the curve and directrix subtends a right angle at the focus. Prove that the locus of the middle points of a system of focal chords of a conic section is a conic section which is a parabola, ellipse or hyperbola according as the original conic is a parabola ellipse or hyperbola. Two equal ellipses of eccentricity e have one focus common and are placed with their axes at right angles. If PQ be a common tangent then prove that sin 1 ∠PSQ = e . 2 2 If the tangents at P and Q of a conic meet at a point T and S be the focus then prove that ST 2 = SP ⋅ SQ if the conic is a parabola. A conic is described having the same focus and eccentricity as the conic l = 1 + e cosq and the two conics touch at q = a . Prove that the length of its r 2l (1 − e 2 ) latus rectum is 2 . e + 2e cos a + 1

6.

7.

8.

9. 10.

11.

12. 13.

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9-52

14.

15.

16.

17.

Analytical Geometry

Prove that three normals can be drawn from a given point to a given parabola. If the normal at a , b ,g on the conic l = 1 + cosq meet at the point ( r ,f ) r a + b +g prove that f = . 2 If the normals at three points of the parabola r = a cos ec 2 q whose vectorial 2 angles are a , b ,g meet in a point whose vectorial angle is f then prove that 2f = a + b + g − p . If a , b ,g be the vectorial angles of three points on 2a = 1 + cosq , the normal r g b at which are concurrent, prove that tan a + tan + tan = 0 . 2 2 2 If the normal at P to a conic cuts the axis in G , prove that SN = eSP .

18.

If SM and SN be perpendiculars from the focus S on the tangent and normal at any point on the conic l = 1 + e cosq and, ST the perpendicular on MN r show that the locus to T is r (e 2 − 1) = el cosq .

19.

Show that if the normals at the points whose vectorial angles are q1 ,q 2 ,q 3 and q 4 on l = 1 + e cosq meet at the point ( r ′,f ) then q1 + q 2 + q 3 + q 4 − 2f = (2n + 1)p . r Prove that the chords of a rectangular hyperbola which subtend a right angle at a focus touch a fixed parabola.

20. 21.

If the tangent at any point of an ellipse make an angle a with its major axis and an angle b with the focal radius to the point of contact then show that Ans.:  A2 + B 2 − 2e( A cosg + B sin g ) + e 2 − 1 = 0   e cos a = cos b . 

22.

Prove that the exterior angle between any two tangents to a parabola is equal to half the difference of the vectorial angles of their points of contact. Find the condition that the straight line l = A cosq + B sinq may be a tangent r to the conic l = 1 − e cos(q − g ). r Find the locus of poles of chords which subtend a constant angle at the focus.

23. 24. 25.

Prove that if the chords of a conic subtend a constant angle at the focus, the tangents at the end of the chord will meet on a fixed conic and the chord will touch another fixed conic.

26.

Find the locus of the point of intersection of the tangents to the conic l = 1 + e cosq at P and Q , where 1 + 1 = 2 , k being a constant. SP SQ k r

27.

If the tangent and normal at any point P of a conic meet the transverse axis of T and G , respectively, and if S be the focus then prove that 1 − 1 is a constant. SG ST

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Chapter

10

Tracing of Curves

10.1  General equation of the second degree and tracing of a conic In the earlier chapters, we studied standard forms of a conic namely a parabola, ellipse and hyperbola. In this chapter, we study the conditions for the general equation of the second degree to represent the different types of conic. In order to study these properties, we introduce the characteristics of change of origin and the coordinate axes, rotation of axes without changing the origin and reducing the second degree equation without xy-term.

10.2  Shift of origin without changing the direction of axes Let Ox and Oy be two perpendicular lines on a plane. Let O′ be a point in the xy-plane. Through O′, draw O′X and O′Y parallel to Ox and Oy, respectively. Let the coordinates of O′ be (h, k) with respect to the axes Ox and Oy. Draw O′L perpendicular to Ox. Then OL = h and O′L = k. y

Y P(x, y) O′

x′

O y′

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(h, k)

X

N

L

M

x

Y′

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10-2

Analytical Geometry

Let P(x, y) be any point in the xy-plane. Draw PM perpendicular to Ox meeting axis at N. Then

x = OM = OL + LM = h + X = X + h



y = MP = MN + NP = LO′ + NP = k + Y = Y + k

10.3  Rotation of axes without changing the origin y X

P(x, y)

Y N

q

M

q q O

L

Q

x

Let Ox and Oy be the original coordinate axes. Let Ox and Oy be rotated through an angle q  in the anticlockwise direction. (i.e.) 

xOX = yOY = q

Let P(x, y) be a point in the xy-plane. Draw PL perpendicular to Ox, PM perpendicular to OX and MN perpendicular to LP. Then NPM = q . Let (X, Y) be the coordinates of the point P with respect to axes OX and OY. Then

OL = OQ - LQ



x = OQ - NM



= OM cosq - MP sinq x = X cosq - Y sinq

(10.1)

LP = LN + NP = QM + NP y = X sinq + Y cosq

(10.2)

From (10.1) and (10.2) we see that X = x cosq + y sinq , Y = −x sinq + y cosq .

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Tracing of Curves

10-3

10.4  Removal of XY-term Here we want to transform the second degree equation ax 2 + 2hxy + by 2 + 22 x + 2 fy + c = 0 into a second degree curve without XYterm, where the axes are rotated through an angle q without changing the origin we get

x = X cosq - Y sinq



y = X sinq + Y cosq ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c

Then

= a( X cos q − Y sin q )2 + 2h( X cos q − Y sin q )( X sin q + Y cos q ) + b( X sin q + Y cos q )2 + 2 g ( X cos q − Y sin q ) + 2 f ( X sin q + Y cos q ) + c = ( a cos 2 q + 2hX sin q cos q + b sin 2 q ) X 2 − 2[( a − b) sin q cos q − h(cos2 q − sin 2 q )] XY + ( a sin 2 q − 2h sin q cos q + b cos 2 q )Y 2 + (2 g cos q + 2 f sin q ) X + (2 f cos q − 2g sin q )Y + c If XY-term has to be absent then Then

( a − b) sin q cos q − h(cos 2 q − sin 2 q ) = 0 tan 2q =

2h a−b

or

q =

1 −1  2h  tan  .  a − b  2

Hence, by rotating the axes through an angle q about O the general second degree expression will result into a second degree expression without XY-terms.

10.5  Invariants We will now prove that the expression ax 2 + 2hxy + by 2 will change to Ax 2 + 2hXY + By 2 if (i) a + b = A + B and (ii) ab - h2 = AB - H 2.

Proof: Let P(x, y) be any point with respect to axes (ox, oy) and (X, Y) be its coordinates with respect to (OX, OY). Then x = X cosq - Y sinq , y = X sinq  + Y cosq . Therefore, we get x2 + y2 = X 2 + Y 2

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10-4

Analytical Geometry

Suppose ax 2 + 2hxy + by 2 = AX 2 + 2 HXY + BY 2 . Then ax 2 + 2hxy + by 2 + l( x 2 + y 2 ) = AX x 2 + 2hxy + by 2 + l ( x 2 + y 2 ) = AX 2 + 2 HXY + BY 2 + d ( X 2 + Y 2 ). If the LHS is of the form (px + qy)2 then it will be changed into the form [ p( X cos q − Y sin q ) + q( X sin q + Y cos q )]2 = ( p1 X + q1Y )2.

LHS = (a + l)2 x2 + 2hxy + (b + l)2 y2

This will be a perfect square if h2 = (a + l) (b + l)

(i.e.)  l 2 + l ( a + h) + ab − h2 = 0 

(10.3)

RHS will be a perfect square if

l 2 + l ( A + B ) + AB − H 2 = 0

(10.4)

Comparing (10.3) and (10.4), we get a + b = A + B, ab - h2 = AB - H 2.

10.6  Conditions for the general equation of the second degree to represent a conic The general equation of the second degree

ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0

(10.5)

If the axes are rotated through an angle q with the anticlockwise direction 2h . then tan 2q = a−b Then the equation transformed with the second degree in (X, Y ) is

AX 2 + BY 2 + 2GX + 2 FY = 0 

(10.6)

where a + b = A + B, ab - h = AB. Now, we study several cases based on the values of A and B. Case 1: If ab - h2 = 0 then A = 0 or B = 0. Suppose A = 0 then the equation (10.6) takes the form 2

2



F CF 2  B  Y +  = −2GX −   B B

(10.7)

If G = 0 then this equation will represent a pair of straight lines. If G ≠ 0 then we have from (3).

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Tracing of Curves

10-5

2

2G  F F2 C   X− +  Y +  = −  2 BG 2G  b B 



 F2 C −F  − , the above equation can be Shifting the origin to the point   2 BG 2G B  2G 1 X which is a parabola. B Case 2: Suppose ab - h2 ≠ 0 then neither A = 0 nor B = 0. Then equation (10.6) can be written as 2

written in the form Y 1 = −

2



2



2

G F G2 F 2   A  X +  + B Y +  = + −C   A B A B 2

G F   A  X +  + B Y +  = K   A B

where K =

G2 F 2 + −C A B

 G F Shifting the origin to the point  − , −  the above equation takes the form  A B

AX 2 + BY 2 = K

(10.8)

If B = 0 then equation (10.8) represents a form of straight lines real or imaginary. If K ≠ 0 then equation (10.8) can be expressed in the form 2



2

X1 Y1 + = 1 ( K / A) ( K /B )

(10.9)

which is an ellipse depending on A and B. If A and B are of opposite signs, that is, ab - h2 < 0 then the equation (10.9) will represent a hyperbola. If B = -A, that is, a + b = 0 then equation (10.9) will represent a rectangular hyperbola. Hence we have the following condition for the nature of the second degree equation (10.3) to represent in different forms. The conditions are as follows: (1) It will represent a pair of straight line if abc + 2fgh - af  2 - bg2 - ch2 = 0. (2) It will represent a circle if a = b and h = 0. (3) It will represent a parabola if ab - h2 = 0. (4) It will represent an ellipse if ab - h2 > 0. (5) It will represent a hyperbola if ab - h2 < 0. (6) It will represent a rectangular hyperbola if a + b = 0.

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10-6

Analytical Geometry

10.7  Centre of the conic given by the general equation of the second degree The general equation of the second degree in x and y is

ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0

(10.10)

Since this equation has x and y terms, the centre is not at the origin. Let us suppose the centre is at (x1, y1). Let us now shift the origin to (x1, y1) without changing the direction of axes. Then X = x + x1, Y = y + y1. Then equation (10.10) takes the form a( X + x1 )2 + 2h( X + x1 )(Y + y1 ) + b(Y + y1 )2 + 2 g ( X + x1 ) + 2 f (Y + y1 ) + c = 0

(i.e.) aX 2 + 2hXY + bY 2 + 2 X ( ax1 + hy1 + g ) + 2Y ( hx1 + fy1 + c) + ax12 + 2hx1 y1 + by12 + 2 gx1 + 2 fy1 + c = 0



(10.11)

Since the origin is shifted to the point (x1, y1) with respect to new axes, the coefficient of X and Y in (10.11) should be zero.

∴ ax1 + hy1 + g = 0

(10.12)



hx1 + by1 + f = 0

(10.13)

Solving these two equations, we get x1 y1 1 = = hf − bg gh − af ab − h2  hf − bg gh − af  Then the coordinates of the centre are  , .  ab − h2 ab − h2  If ab - h2 = 0, then the conic is a parabola.

10.8  Equation of the conic referred to the centre as origin From the result obtained in Section 10.7, the equation of the ­conic ­referred to centre as the origin is ax 2 + 2hxy + by 2 + C1 = 0 , where,

C1 = ax12 + 2hx1 y1 + by12 + 2 gx1 + 2 fy1 + c = x1 ( ax1 + by1 + g ) + y1 ( hx1 + by1 + f ) + gx1 + fy1 + c

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Tracing of Curves



10-7

= gx1 + fy1 + c [Using (10.12) and (10.13) of Section 10.8]  hf − bg   gh − af  = g +f +c 2   ab − h   ab − h2  =

abc + 2 fgh − af 2 − bg 2 − ch2 ∆ = ab − h2 ab − h2

Hence, the equation of the conic referred to centre as origin is ∆ ax 2 + 2hxy + by 2 + C1 = 0 where C1 = . ab − h2

Note 10.8.1: If f = ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0 then



∂f = 2( ax + hy + g ) ∂x ∂f = 2( hx + by + f ) ∂y

Therefore, the coordinates of the centre of the conic are given by solving the ∂f ∂f = 0 and = 0. equations ∂x ∂y

Example 10.8.1 Find the nature of the conic and find its centre. Also write down the equation of the conic referred to centre as origin: (i) 2 x 2 − 5 xy − 3 y 2 − x − 4 y + 5 = 0 (ii) 5 x 2 − 6 xy + 5 y 2 + 22 x − 26 y + 29 = 0

Solution (i) Given: 2 x 2 − 5 xy − 3 y 2 − x − 4 y + 5 = 0 −5 Here,  a = 2, b = −3, h = 2 25 ab − h2 = −6 − < 0. Therefore, the conic is a hyperbola. 4 ∂f ∂f = 0, =0 The coordinates of the centre are given by ∂x ∂y \ 4x - 5y - 1 = 0 –5x - 6y - 4 = 0

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10-8

Analytical Geometry

−2 −3 ,y= . 7 7  −2 −3  Therefore, the coordinates of the centre are  ,  . The equation of  7 7 the ellipse referred to centre as origin is Solving these two equations, we get x =

2 x 2 − 5 xy − 3 y 2 + C1 = 0

where C1 = gx1 + fy1 + c −1  −2  4  −3    −   +6 2  7  2 7  1 6 = + +6 = 7 7 7

=



Therefore, the equation of the ellipse referred to the centre is 2x2 5xy - 3y2 + 7 = 0. (ii) 5x2 - 6xy + 5y2 + 22x - 26y + 29 = 0 a = 5, b = 5, h = 3, ab - h2 = 25 - 9 = 16 > 0

(10.14)

Therefore, the given equation represents an ellipse. The coordinates of ∂f ∂f = 0, = 0. the centre are given by ∂x ∂y 10x - 6y + 22 = 0 -6x + 10y - 26 = 0 Solving these equations we get the centre as (1, 2). The equation of the conic referred to centre as origin is 5x2 - 6xy + 5y2 + C1 = 0 where C1 = gx1 + fy1 + c.

C1 = 11 × (-1) - 13(2) + 29 = −11 - 26 + 29 = -8

Therefore, the equation of the ellipse is 5x2 - 6xy + 5y - 8 = 0 or 5x2 - 6xy + 5y = 8.

10.9  Length and position of the axes of   the central conic whose equation is   ax 2 + 2hxy + by 2 = 1 Given:

ax2 + 2hxy + by2 = 0

(10.15)

The equation of the circle concentric with this conic and radius r is

M10_ANALYTICALGEOMETRY_ISBN_CH10.indd 8

x2 + y2 = r2

(10.16)

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Tracing of Curves

10-9

Homogeneousing equation (10.15) with the help of (10.16) we get

ax 2 + 2hxy + by 2 =

x2 + y2 r2

1 1   (i.e.)   a − 2  x 2 + 2hxy +  b − 2  y 2 = 0   r  r 

(10.17) (10.18)

The two lines given by above homogeneous equation will be considered only if the radius of the circle is equal to length of semi-major axis or semiminor axis. The condition for that is

1 1  2  a − 2   b − 2  = h r r

(10.19)

This is a quadratic equation in r2 and so it has two roots namely r12 and r22. For an ellipse the values r12 and r22 are both positive and the lengths of the semi-axes are 2r1 and 2r2 . For a hyperbola one of the values is positive and the other is negative, that is, say r12 is positive and r22 is negative. Then the length of transverse axis is 2r1 and the length of conjugate axis is 2 −r22 . Using equation (10.18) in equation (10.17), we get   1 2  h2  2  y =0  a − 2  x + 2hxy +  1 r a− 2   r  2



1 1 2   2 2  a − 2  x + 2h  a − 2  xy + h y = 0 r r 2

  1 (i.e.)  a − 2  x + hy  = 0 r    Then the equations of axes are



 1  a − r 2  x + hy = 0 1

 1  a − r 2  x + hy = 0 2

The eccentricity of the conic can be determined from the length of the axes.

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10-10

Analytical Geometry

10.10  Axis and vertex of the parabola whose equation is ax 2 + 2hxy + by 2 + 2gx + 2fy + c = 0 This equation will represent a parabola if ab - h2 = 0. Given:

ax2 + 2hxy + by2 + 2gx + 2fy + c = 0.

(10.20)

Then equation (10.20) can be expressed in the form ( px + qy + l)2 + 2gx + 2fy + c = 0 (i.e.) 

(px + qy - l)2 + 2(lp - g)x + 2(l q - f  )y + l2 + c = 0  

We choose l such that px + qy + l = 0 and 2(l p - g)x + 2f (l q - f  )y + l2 - c = 0

(10.21)

(10.22) (10.23)

are perpendicular to each other.

 − p  l p − g   q   − lq − f  = −1 l=

or

pg + rf p2 + q2

Now equation (10.21) can be written as 1   2 ( l p − q) x + ( l q − f ) y + ( l 2 − c )  px + qy + l    2 2 2   ( p + q ) = 2  2 2 2 2 ( l p − g ) + ( lq − f )  p +q    × ( l p − g )2 + ( lq − f )2 2



 px + qy + l  2 ( l p − q) 2 + ( l q − f ) 2 = (i.e.)   2 2 p2 + q2  p +q 

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1   ( l p − q) x + ( l q − f ) y + ( l 2 − c )   2   2 2 ( l p − g ) + ( lq − f )  

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Tracing of Curves

10-11

Since the above equation represents a parabola, the axis of the parabola is px + 1 qy + l = 0 and the tangent at the vertex is ( l p − q) x + ( l q − f ) y + ( l 2 − c) = 0 2 and the length of the latus rectum is pg + q f l= 2 . p + q2

2 ( l p − q) 2 + ( l q − f ) 2 p2 + q2

where

Example 10.10.1 Trace the conic 36x2 + 24xy + 29y2 - 72x + 126y + 81 = 0.

Solution Given:

36x2 + 24xy + 29y2 - 72x + 126y + 81 = 0

(10.24)

a = 36, b = 29, h = 12



ab - h2 = 36 × 29 - 144 > 0



Therefore, the given conic represents an ellipse. The coordinates of the centre ∂f ∂f = 0 and = 0. are given by ∂x ∂y



72x + 24y - 72 = 0  or  3x + y - 3 = 0

(10.25)

24x + 58y + 126 = 0  or  12x + 29y + 63 = 0

(10.26)

Solving equation (10.25) and (10.26) we get x = 2, y = −3. Therefore, the centre of the ellipse is (2, −3). The equation of this ellipse in standard form is 36x2 + 24xy + 29y2 + C1 = 0 where C1 = gx1 + fy1 + c. C1 = −36(2) + 63(−3) + 81 = −180.



\ 36x2 + 24xy + 29y2 = 180 (i.e.)

36 2 24 29 2 x + xy + y =1 180 180 180

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10-12

Analytical Geometry

The length of the axes are given by 1 1  2  a − 2   b − 2  = h r r



1 1   36 − 2   29 − 2  = 144 r r

Solving for r2 we get r2 = 9 or 4 \ r1 = 3 = Length of the semi-major axis r2 = 2 = Length of the semi-minor axis 1  The equation of the major axis is  a − 2  x + hy = 0 .  r  12  36 20  (i.e.)  − x+ y = 0 or 4 x + 3 y = 0.  180 180  180



45  12  36 The equation of minor axis is  − x+ y = 0 or 3 x − 4 y = 0.  180 180  180 Referring to the centre the equation of major and minor axes are 4(x - 2) + 3(y + 3) = 0 3(x - 2) - 4(y + 3) = 0



(i.e.)  4x + 3y + 1 = 0 and 3x - 4y - 18 = 0 1   1  The major axis 4x + 3y + 1 = 0 meets the axes at  0, −  and  − , 0 .   4  3  9  The minor axis meets the axes at the point  − , 0 and (6, 0).  2  The conic meets the axis at points are given by

36x2 - 72x + 81 = 0



(i.e.)  4x2 - 8x + 9 = 0

8 ± 64 − 4 × 4 × 9 which are imaginary. 2×4 Therefore, the conic does not meet the x-axis. Similarly, by substituting y = 0 in equation (10.24) we get 29y2 + 126y + 81 = 0.





x=

y=

−126 ± 1262 − 4 × 29 × 81 = (0, 3.85),(0, 0.49) 58

Therefore, the conic intersects y-axis in real points.

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Tracing of Curves

10-13

Example 10.10.2 Trace the conic x2 + 4xy + y2 - 2x + 2y - 6 = 0.

Solution ab - h2 = 1 × 1 - 4 = −3 < 0.



Therefore, the conic is a hyperbola. The coordinates of the centre are given ∂f ∂f = 0, = 0. by ∂x ∂y

(i.e.)  2x + 4y - 2 = 0  or  x + 2y - 1 = 0

4x + 2y + 2 = 0  or  2x + y + 1 = 0. Solving these two equations we get the centre. Therefore, the coordinates of the centre are (−1, 1). The equation of the conic referred to the centre as origin without changing the directions of the axis is x2 + 4xy + y2 + C1 = 0 where C1 = gx + fy + c = (−1) × (−1) + 1 × 1 - 6 = -4



x2 + 4yx + y2 = 4 (i.e.)

x2 y2 + xy + =1 4 4

Therefore, the lengths of the axes are given by



 1 1   1 1   1  − 2   − 2  =   4 r 4 r 2 or

2

1 1 1 4 − 2 =± or r 2 = or − 4 4 r 2 3

Hence, the semi-transverse axis is r =

2 3

.

The length of semi-conjugate axis is r22 = 2. Semi-latus rectum =

r22 = 2 3. r1

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10-14

Analytical Geometry

The equation of the transverse axis is 1 1  1  4 − r 2  ( x + 1) + 2 ( y − 1) = 0 1

1  1 3 (i.e.)  −  ( x + 1) + ( y − 1) = 0  4 4 2



( x + 1) − ( y − 1) = 0 (i.e.) x − y + 2 = 0 1  1 1 The equation of conjugate axes is  +  ( x + 1) + ( y + 1) = 0 .  4 4 2 (i.e.)  x + y = 0 The points where the hyperbola meets the x-axis are given by x2 - 2x - 6 = 0. 2 ± 4 + 24 = −1.7 or 3.7 nearly 2 4 Eccentricity is given by 4 = (e 2 − 1) ∴e = 2 3 4 So CS = CS′ = Ge = . 3 When the curve meets the y-axis, x = 0 \ y2 + 2y - 6 = 0 \ y = 1.7 or −3.7 nearly Hence, the curve passes through the points (−1.7, 0), (3.7, 0), (0, 1.7) and (0, −3.7). The curve is traced in following figure: x=



y

i

C ) ,1 (−1

S

x S′

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Tracing of Curves

10-15

Example 10.10.3 Trace the conic x2 + 2xy + y2 - 2x - 1 = 0.

Solution a = 1, b = 1, h = 1, g = −1, f = 0, c = −1 2 Here, h = ab and abc + 2fgh –af  2 - bg2 - ch2 ≠ 0. Therefore, the conic is a parabola. The given equation can be written as (x + y)2 = 2x + 1. The equation can be written as

(x + y + 1)2 = 2(l + 1) + 2ly + (l2 + 1) = 0

(10.27)

where l is chosen such that x + y + l = 0 and 2(l + 1)x + 2ly + (l + 1) = 0 are perpendicular. 2



1  −2( l + 1)  ∴ ( −1)  = −1 or l + 1 = − l or l = − .  2  2l  2

1 5   Now equation (10.27) can be written as  x + y −  =  x − y +    2 4 2

1  x+ y−  2  = 1  x − y + 5 (i.e.)     2  2 2     2 which is of the form y = 4ax. 1 Therefore, lengths of latus rectum of the parabola is . 2 1 The axis of the parabola is x + y − = 0 2 or 2x + 2y - 1 = 0 The equation of the tangent at the vertex is

x− y+

5 = 0 or 4 x − 4 y + 5 = 0 4

 3 7 Vertex of the parabola is  − ,  .  8 8 When the parabola meets the x-axis, y = 0.

x2 - 2x - 1 = 0  \ x = 2.4, −0.4

Therefore, the points on the curve are (−0.4, 0) and (2.4, 0).

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10-16

Analytical Geometry

When the curve meets the y-axis, x = 0. y2 = 1 or y = ±1



(−

3/

8,

7/

8)

Hence (0, −1) and (0, 1) are points on the curve. The graph of the curve is given as follows:

Exercises Trace the following conics:

1. 2. 3. 4. 5. 6. 7. 8. 9.

9x2 + 24xy + 16y2 - 44x + 108y - 124 = 0 5x2 - 6xy + 5y2 + 22x - 26y + 29 = 0 32x2 + 52xy - 7y2 - 64x - 52y - 148 = 0 x2 + 24xy + 16y2 - 86x + 52y - 139 = 0 43x2 + 48xy + 57y2 + 10x + 180y + 25 = 0 x2 - 4xy + 4y2 - 6x - 8y + 1 = 0 x2 + 2xy + y2 - 4x - y + 4 = 0 5x2 - 2xy + 5y2 + 2x - 10y - 7 = 0 22x2 - 12xy + 17y2 - 112x + 92y + 178 = 0.

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Chapter

11

Three Dimension

11.1  Rectangular Coordinate Axes To locate a point in a plane, two numbers are necessary. We know that any point in the xy plane can be represented as an ordered pair (a, b) of real numbers where a is called x-coordinate of the point and b is called the y-coordinate of the point. For this reason, a plane is called two dimensional. To locate a point in space, three numbers are required. Any point in space is represented by an ordered triple (a, b, c) of real numbers. To represent a point in space we first choose a fixed point ‘O’ (called the origin) and then three directed lines through O which are perpendicular to each other (called coordinate axes) and labelled x-axis, y-axis as being horizontal and z-axis as vertical and we take the orientation of the axes. In order to do this, we first choose a fixed point O. In looking at the figure, you can think of y- and zaxes as lying in the plane of the paper and x-axis as coming out of the paper towards y-axis. The direction of z-axis is determined by the neighbourhood rule. If you curl the fingers of your right-hand around the z-axis in the direction of a 90° counter clockwise rotation from the positive x-axis to the positive y-axis then your thumb points in the positive direction of the z-axis. z

O

y

x

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11-2

Analytical Geometry

The three coordinate axes are determined by the three coordinate planes. The xy-plane is the plane that contains x and y-axes, the yz-plane contains y and z-axes and the xz-plane contains x- and z-axes. z

xz-plane

yz-plane

y xy-plane x

These three coordinate planes divide the space into eight parts called octants. The first octant is determined by the positive axes. Look at any bottom corner of a room and call the corner as origin.

right wall

left wall

floor

The wall on your left is in the xz-plane. The wall on your right is in the yz-plane. The wall on the floor is in the xy-plane. The x-axis runs along the intersection of the floor and the left wall. The y-axis runs along the intersection of the floor and the right wall. The z-axis runs up from the floor towards the ceiling along the intersection of the two walls are situated in the first octant and you can now imagine seven other rooms situated in the other seven octants (three on the same floor and fourth on the floor below), all are connected by the common point O.

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Three Dimension

11-3

If P is any point in space, and a be the directed distance in the first octant from the yz-plane to P. Let the directed distance from the xz-plane be b and let c be the distance from xy-plane to P. y

P(a, b, c)

O

x

c

a b z

We represent the point P by the ordered triple (a, b, c) of real numbers and we call a, b and c the coordinates of P. a is the x-coordinate, b is the y-coordinate and c is the z-coordinate. Thus, to locate the point (a, b, c) we can start at the origin O and move a units along x-axis then b units parallel to y-axis and c units parallel to the z-axis as shown in the above figure. y C

(0, 0, c)

A′(0, b, c)

P (a, b, c)

B′ (a, 0, c)

B

O

(0, b, 0) z

A x

(a, 0, 0)

C ′(a, b, 0)

The point P(a, b, 0) determines a rectangular box as in the above figure. If we drop a perpendicular from P to the xy-plane we get a point C′ with coordinates P(a, b, 0) called the projection of P on the xy-plane. Similarly, B′ (a, 0, c) and A′(0, b, c) are the projections of P on xz-plane and yz-plane, respectively. The set of all ordered triples of real numbers is the Cartesian product R × R × R = {( x, y, z ) x, y, z ∈ R} R × R = {( x, y, z ) x, y, z ∈ R}, which is R3. We have a one-to-one correspondence between the points P in space and ordered triples (a, b, c) in R3. It is called a three-­ dimensional rectangular coordinate system. We notice that in terms of coordinates, the first octant can be described as the set {( x, y, z ) x ≥ 0, y ≥ 0, z ≥ 0}.

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11-4

Analytical Geometry

11.2  Formula for distance between two points z Q

(x2, y2, z2)

P(x, y, z) A(x1, y1, z1)

B(x2, y2, z2) y

x

Consider a rectangular box, where P and Q are the opposite corners and the faces of the box are parallel to the coordinate planes. If A(x1, y1, z1) and B(x2, y2, z2) are the vertices of the box indicated in the above figure, then PA = x2 − x1 , AB = y2 − y1 , BQ = z2 − z1



Since the triangles PBQ and PAB are both right angled, by Pythagoras theorem, (PQ)2 = (PB)2 + (BQ)2 and (PB)2 = (PA)2 + (PB)2

2

2

2

2

∴ PQ = PA + AB + BQ = ( x2 − x1 )2 + ( y2 − y1 )2 + ( z2 − z1 )2 ∴ PQ = ( x2 − x1 )2 + ( y2 − y1 )2 + ( z2 − z1 )2 = ( x1 − x2 )2 + ( y1 − y2 )2 + ( z1 − z2 )2

Example 11.2.1 Find the distance between the points (2, 1, −5) and (4, −7, 6).

Solution The distance between the points (2, 1, −5) and (4, −7, 6) is PQ = (2 − 4)2 + (1 + 7)2 + ( −5 − 6)2 = 4 + 64 + 121 = 189 units

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Three Dimension

11-5

Aliter: Let P(x1, y1, z1) and (x2, y2, z2) be two points. Draw PA, QB perpendicular to xy-plane. Then the coordinates of A and B are (x1, y1, 0) and (x2, y2, 0).

(i.e.)  (x1, y1) and (x2, y2) in the xy-plane. z (x1, y1, z1)

Q (x2, y2, z2) C

P

y A

B

x

\ AB2 = (x2 − x1)2 + (y2 − y1)2 Draw PC perpendicular to xy-plane. PA and PB being perpendicular to xy-plane, PA and QB are also perpendicular to AB. \ PABC is a rectangle and so PC = AB and PA = CB. From triangle PCQ, PQ2 = PC 2 + CQ2 = MN 2 + (AQ − AC)2

= MN 2 + (CQ − PA)2



= (x2 − x1)2 + (y2 − y1)2 + (z2 − z1)2 ∴ PQ = ( x2 − x1 ) 2 + ( y2 − y1 )2 + ( z2 − z1 )2



Example 11.2.2 If O is the origin and P is the point (x, y, z) then OP2 = x2 + y2 + z2 or OP =

x2 + y2 + z2.

11.2.1  Section Formula The coordinates of a point that divides the line joining the points (x1, y1, z1) and (x2, y2, z2) are in the ratio l:m.

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11-6

Analytical Geometry

Let R(x, y, z) divide the line joining the points P(x1, y1, z1) and Q(x2, y2, z2) in the ratio l:m. y Q R

L

M

P

O

z

C B A x

Draw PL, QC and RB perpendicular to the xy-plane. Then ACB is a straight line since projection of a straight line on a plane is a straight line. Through R draw a straight line LRM parallel to ACB to meet AP (produced) in A and CQ in M. Then triangles LPR and MRQ are similar. PL PR l = =  MQ RQ m



(11.1)

However, PL = LA − AP = BR − AP = z − z1

MQ = CQ − CM = CQ − BR = z2 − z Therefore, from (11.1),

z − z1 l = ⇒ m( z − z1 ) = l ( z2 − z ) z2 − z m

mz − mz1 = lz2 − lz ⇒ (l + m) z = lz2 + mz1

⇒z=

lz2 + m1 z1 l+m

Similarly, by drawing perpendiculars on xz and yz planes we can prove that ly + my1 lx + mx1 y= 2 ,x = 2 . l+m l+m  lx + mx1 ly2 + my1 lz2 + mz1  , , . Therefore, the coordinates of R are  2 l+m l + m   l+m

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Three Dimension

11-7

Note 11.2.1.1: If R′ divides PQ externally in the ratio l:m then,

PR ′ = R ′Q

l PQ ′ l . or = m QR ′ − m l P

Q

m

R′

∴Therefore, change m into −m to get the coordinates of R′, the external point of division. The coordinates of external point of division are  lx2 − mx1 ly2 − my1 lz2 − mz1   l−m , l−m , l−m .  

Note 11.2.1.2: To find the midpoint of PQ take l:m = 1:1.  x + x y + y2 z1 + z2 , Then the coordinates of midpoint are  1 2 , 1 2 2  2

 . 

11.3  CENTROID OF TRIANGLE Let ABC be a triangle with vertices A(x1, y1, z1), B(x2, y2, z2) and C(x3, y3, z3).  x + x3 y2 + y3 z2 + z1  Then the midpoint of BC is D  2 , , . 2 2   2 A

F B

E D

C

Let G be the centroid of the triangle ABC. Then G divides AD in the ratio 2:1. Then the Coordinates of G are



  x2 + x3    y2 + y3   z2 + z3   2  2  + 1⋅ x1 2  2  + 1⋅ y1 2  2  + 1⋅ z1   . , ,   2 +1 2 +1 2 +1  x + x + x3 y1 + y2 + y3 z1 + z2 + z3  (i.e.)   1 2 , ,  3 3 3  

 x + x + x3 y1 + y2 + y3 z1 + z2 + z3  Hence, the centroid of ∆ABC is  1 2 , , . 3 3 3  

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11-8

Analytical Geometry

11.4  Centroid of Tetrahedron Let OBCD be a tetrahedron with vertices (xi, yi, zi), i = 1, 2, 3, 4. A

G D

B

C

The centroid of the tetrahedron divides AD in the ratio 3:1. Therefore, the coordinates of G are

 x1 + x2 + x3 + x4 y1 + y2 + y3 + y4 z1 + z2 + z3 + z4  , ,   4 4 4

11.5  DIRECTION COSINES Direction cosines in three-dimensional coordinate geometry play a role similar to slope in two-dimensional coordinate geometry.

Definition 11.5.1: If a straight line makes angles a, b  and g  with the positive directions of x-, y- and z-axes then cosa, cosb and cosg  are called the direction cosines of the line. The directional cosines are denoted by l, m and n.

\ l = cosa, m = cosb, n = cosg.

The direction cosines of x-axis are 1, 0 and 0. The direction cosines of y-axis are 0, 1 and 0. The direction cosines of z-axis are 0, 0 and 1. If O is the origin and P(x, y, z) be any point in space and OP = r, then the direction cosines of the line are lr, mr, nr. Let O be the origin and P(x, y, z) is any point in space. Draw PN perpendicular to XOY plane. Draw NL, NM parallel to y- and x-axes.

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Three Dimension

11-9

z

P(x, y, z) g O

M

b a

L

y

N

x

Then OL = x1, OM = y1, PN = z1. OM Also, = cosb (i.e.) y = r cosb  OP Similarly, x = r cosa and z = r cosg. Then the coordinates of P are (r cosa, r cosb, r cosg) (i.e.)

(lr, mr, nr).

x y z r r r Note 11.5.3: If OP = 1 unit then the direction cosines of the line are (x, y, z). That is, the coordinates of the point P are the same as the direction cosines of the line OP.

Note 11.5.2: The direction cosines of the line OP are , , .

Note 11.5.4: If OP = 1 unit and P is the point (x, y, z) then OP2 = x2 + y2 + z2

or x2 + y2 + z2 = 1. \ l 2 + m2 + n2 = 1 Therefore, direction cosines satisfy the property cos2a + cos2b + cos2g = 1.

11.5.1  Direction Ratios Suppose a, b and c are three numbers proportional to l1, m1 and n1 (the direction cosines of a line), then

l m n l 2 + m 2 + n2 1 = = =± =± 2 2 2 2 a b c a +b +c a + b2 + c2 c a b ,n = ± ,m = ± ∴l = ± 2 2 2 2 2 2 2 a + b2 + c2 a +b +c a +b +c

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11-10

Analytical Geometry

a Therefore, the direction cosines of the line are ± , 2 a + b2 + c 2 b c ± ,± where the same sign is taken throughout. a2 + b2 + c 2 a2 + b2 + c2 Here a, b and c are called the direction ratios of the line. If 2, 3 and 5 are the ­direction ratios of a line then the direction cosines of the line are    ±2 ±5 2 3 5  3 ,± , ,± ,± .   (i.e.)  ± 38 38 38  4 + 9 + 25 4 + 9 + 25    4 + 9 + 25

11.5.2  Projection of a Line The projection of a line segment AB on a line CD is the line joining the feet of the perpendiculars from A and B on CD. If AL makes an angle p with the line CD then LAB = q , where AL is parallel to CD. B

q

A

C

L

L

M

D

AL = AB cosp LM = AB cosp



Therefore, the projection of AB on CD is LM = AB cosp.

Note 11.5.2.1: The projection of broken lines AB, BC and CD on the line CD is LM, MN and ND. A

C

D

N

P D

B

C

L

M

∴Therefore, the sum of the projection AB, BC and CD is LM + MN + ND = LP.

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Three Dimension

11-11

11.5.3  Direction Cosines of the Line Joining Two Given Points Let P(x1, y1, z1) and Q(x2, y2, z2) be the given points. We easily see that the ­projection of PQ on x-, y- and z-axes are x2 − x1, y2 − y1 and z2 − z1. However, the projections of PQ on x-, y- and z-axes are also PQcosa, PQcosb and PQcosg. x −x \ PQ ⋅ l = x2 − x1 \l= 2 1 PQ y2 − y1 z −z , n = 2 1 . Since PQ is of constant length, the PQ PQ direction ratios of PQ are

In addition, m =

(x2 − x1, y2 − y1, z2 − z1).



11.5.4  Angle between Two Given Lines Let (l1, m1, n1), (l2, m2, n2) be the direction cosines of the two given lines namely AB and CD. Through O draw lines parallel to AB and CD and take points P and Q such that OP = OQ = 1 unit. B z

D A C

Q

P q O

y

x

Since OP and OQ are parallel to the two given lines then the angle between the two given lines is equal to the angle between the lines OP and OQ. Since OP = OQ = 1 unit, the coordinates of P and Q are (l1, m1, n1) and (l2, m2, n2). Let POQ = q .

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11-12

Analytical Geometry

Then PQ2 = OP2 + OQ2 − 2⋅OP · OQ cosq = 1 + 1 − 2⋅1⋅1 cosq PQ2 = 2 − 2cosq



(11.2)

Also, PQ2 = (l2 − l1)2 + (m2 − m1)2 + (n2 − n1)2 = (l12 + m12 + n12 ) + (l22 + m22 + n22 ) − 2(l1l2 + m1m2 + n1n2 )



PQ2 = 2 − 2(l1l2 + m1m2 + n1n2)

(11.3)

From (11.2) and (11.3), 2 − 2cosp = 2 − 2(l1l2 + m1m2 + n1n2) cosp = l1l2 + m1m2 + n1n2



(11.4)

Note 11.5.4.1: If the two lines are perpendicular then q  = 90° and cos90° = 0.

\ from (11.3), l1l2 + m1m2 + n1n2 = 0

Note 11.5.4.2: sinp = 1 − cos 2 q = (l12 + m12 + n12 )(l2 2 + m2 2 + n2 2 ) − (l1l2 + m1m2 + n1n2 )2



= (l1m2 − l2 m1 )2 + ( m1n2 − m2 n1 )2 + ( n1l2 − n2 l1 )2 =

Note11.5.4.3: tanp =

∑ (m n

1 2

− m2 n1 )2

sin θ ∑ (m1n2 − m2 n1 )2 = l1l2 + m1m2 + n1n2 cos θ

Note 11.5.4.4: If a1, b1, c1 and a2, b2, c2 are the direction ratios of the two lines then cosp = sinp =

a1a2 + b1b2 + c1c2

a + b12 + c12 a2 2 + b2 2 + c2 2 2 1

∑ (a b ∑a

1 2 2 1

− a2 b1 )2 a2 2

, tanp =

.

∑ (a b

1 2

− a2 b1 )2

a1a2 + b1b2 + c1c2

If the two lines are parallel then sinp = 0. \ ∑ ( m1n2 − m2 n1 )2 = 0.

∴ (l1m2 − l2 m1 )2 + ( m1n2 − m2 n1 )2 + ( n1l2 − n2 l1 )2 = 0. l1m2 − l2 m1 = 0



m1n2 − m2 n1 = 0 n1l2 − n2 l1 = 0

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Three Dimension

∴

11-13

l1 m1 = l2 m2

m1 n1 = m2 n2



n1 l1 = n2 l2 l1 m1 n1 = = l2 m2 n2 Also, if a1, b1, c1 and a2, b2, c2 are the direction ratios of two parallel lines then a1 b1 c1 = = . a2 b2 c2 ∴



ILLUSTRATIVE EXAMPLES Example 11.1 Show that the points (−2, 5, 8), (−6, 7, 4) and (−3, 4, 4) form a right-angled triangle.

Solution The given points are A(−2, 5, 8), B(-6, 7, 4) and C(−3, 4, 4). AB 2 = ( x1 − x2 )2 + ( y1 − y2 )2 + ( z2 − z1 )2 = ( −2 + 6)2 + (5 − 7)2 + (8 − 4)2

= 16 + 4 + 16 = 36 BC = ( −6 + 3)2 + (7 − 4)2 + ( 4 − 4)2 + 0 = 9 + 9 = 18 2

AC 2 = ( −2 + 3)2 + (5 − 4)2 + (8 − 4)2 = 1 + 1 + 16 = 18 Since BC 2 + AC 2 = AB2, the triangle is right angled. Since BC = AC, the triangle is also isosceles.

Example 11.2 Show that the points (3, 2, 5), (2, 1, 3), (−1, 2, 1) and (0, 3, 3) taken in order form a parallelogram.

Solution Let the four points be A(3, 2, 5), B(2, 1, 3), C(−1, 2, 1) and D(0, 3, 3).

M11_ANALYTICALGEOMETRY_ISBN_CH11.indd 13

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11-14

Analytical Geometry

Then,

AB 2 = (3 − 2)2 + (2 − 1)2 + (5 − 3)2 = 1 + 1 + 4 = 6. BC 2 = (2 + 1)2 + (1 − 2)2 + (3 − 1)2 = 9 + 1 + 4 = 14. CD 2 = ( −1 − 0)2 + (2 − 3)2 + (1 − 3)2 = 1 + 1 + 4 = 6. AD 2 = (3 − 0)2 + (2 − 3)2 + (5 − 3)2 = 9 + 1 + 4 = 14.

Since AB = CD and BC = AD, the four points form a parallelogram.

Aliter: The midpoint of AC is (1, 2, 3). The midpoint of BD is (1, 2, 3). Therefore, in the figure ABCD, the diagonals bisect each other. Hence ABCD is a parallelogram.

Example 11.3 Show that the points (−1, 2, 5), (1, 2, 3) and (3, 2, 1) are collinear.

Solution The three given points are A(−1, 2, 5), B(1, 2, 3) and C(3, 2, 1). AB 2 = ( −1 − 1)2 + (2 − 2)2 + (5 − 3)2 = 4 + 0 + 4 = 8

BC 2 = (1 − 3)2 + (2 − 2)2 + (3 − 1)2 = 4 + 0 + 4 = 8 AC 2 = ( −1 − 3)2 + (2 − 2)2 + (5 − 1)2 = 16 + 0 + 16 = 32

∴ AB = 2 2 , BC = 2 2 and AC = 4 2 \AB + BC = AC Hence, the three given points are collinear.

Example 11.4 Show that the points (3, 2, 2), (−1, 1, 3), (0, 5, 6) and (2, 1, 2) lie on a sphere whose centre is (1, 3, 4). Also find its radius.

Solution Let the given points be S(2, 1, 2), P(3, 2, 2), Q(−1, 1, 3), R(0, 5, 6) and C(1, 3, 4). CP 2 = (1 − 3)2 + (3 − 2)2 + ( 4 − 2)2 = 4 + 1 + 4 = 9 CQ 2 = (1 + 1)2 + (3 − 1)2 + ( 4 − 3)2 = 4 + 4 + 1 = 9

CR2 = (1 − 0)2 + (3 − 5)2 + ( 4 − 6)2 = 1 + 4 + 4 = 9 CS 2 = (1 − 2)2 + (3 − 1)2 + (44 − 2)2 = 1 + 4 + 4 = 9 ∴ CP = CQ = CP = CS = 9 = 3.

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Three Dimension

11-15

Therefore, the points P, Q, R and S lie on a sphere whose centre is C(1, 3, 4) and whose radius is 3 units.

Example 11.5 Find the ratio in which the straight line joining the points (1, −3, 5) and (7, 2, 3) is divided by the coordinate planes.

Solution Let the line joining the points P(1, −3, 5) and Q(7, 2, 3) be divided by XY, YZ and ZX planes in the ratio l:1, m:1 and n:1, respectively. When the line PQ meets the XY planes, the Z-coordinates of the point of meet is 0.

\

7l + 1 =0 l +1

∴ 7l + 1 = 0 or l =



−1 . 7

(i.e.) The ratio in which PQ divides the plane YZ-plane is 1:7 externally. 2m − 3 Similarly, = 0. 7m + 1 3 ∴ 2m − 3 = 0 or m = 2 Since XZ-plane divides PQ in the ratio 3:2 internally. 3n + 5 −5 Also = 0 or n = . Therefore, XY-plane divides PQ in the ratio 5:3 3 n +1 externally.

Example 11.6 P and Q are the points (3, 4, 12) and (1, 2, 2). Find the coordinates of the points in which the bisector of the angle POQ meets PQ.

Solution O (0, 0, 0)

P (3, 4, 12)

13

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R

3

Q (1, 2, 2)

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11-16

Analytical Geometry



QP = 9 + 16 + 144 = 169 = 13

OQ = 1 + 4 + 4 = 9 = 3 PR 13 We know that = RQ 3

R divides PQ internally in the ratio 13:3 and S divides PQ externally in the ratio 13:3.  13 × 1 + 3 × 3 26 + 12 26 + 24  , , Therefore, the coordinates of R are  .  16 16 16   11 19 25  8,8, 8   

(i.e.)

S divides PQ externally in the ratio 13:3.  13 − 9 26 − 12 26 − 24  , , . Therefore, the coordinates of S are  10 10   10 2 7 1 (i.e.) 5,5,5  

Example 11.7 Prove that the three lines which join the midpoints of opposite edges of a tetrahedron pass through the same point and are bisected at that point.

Solution A

D

B

C

Let ABCD be a tetrahedron with vertices ( xi , yi , zi ) , i = 1, 2, 3, 4. The three pairs of opposite edges are (AD, BC), (BD, AC) and (CD, AB). Let (L, N), (P, Q) and (R, S) be the midpoints of the three pairs of opposite edges. Then L is the  x + x3 y2 + y3 z2 + z3   x + x y + y4 z1 + z4  , point  1 4 , 1 , M is the point  2 , , .  2 2  2 2   2  2

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Three Dimension

11-17

 x + x + x3 + x4 y1 + y2 + y3 + y4 z1 + z2 + z3 + z4  The midpoint of LM is  1 2 , , . 4 4 4   By symmetry, this is also the midpoint of the lines PQ and RS. Therefore, the lines LM, PQ and RS are concerned and are bisected at that point.

Example 11.8 A plane triangle of sides a, b and c is placed so that the midpoints of the sides are on the axes. Show that the lengths l, m and n intercepted on the axes are given by 8l 2 = b 2 + c 2 − a 2, 8m2 = c 2 + a 2 − b 2 and 8n2 = a 2 + b 2 − c 2 and that the coordinates of the vertices of the triangle are (−l, m, n), (l, −m, n) and (l, m, −n).

Solution z

B(x2, y2, z2)

l

D(l, 0, 0)

F(0, 0, n) A(x , y , z ) 1 1 1

n o m

y E(0, m, 0)

x

C(x3, y3, z3)

Let D, E and F be the midpoints of the sides BC, CA and AB, respectively. D, E and F are the points (l, 0, 0), (0, m, 0), (0, 0, n), respectively. Let A, B and C be the points ( x1 , y1 , z1 ), ( x2 , y2 , z2 ) and ( x3 , y3 , z3 ), respectively. Then,

x2 + x3 2 x3 + x1 2 x1 + x2 2 x1 + x2 + x3

y2 + y3 z +z = 0, 2 3 = 0 2 2 y3 + y1 z3 + z1 = 0, = m, =0 2 2 y + y2 z +z = 0, 1 = 0, 1 2 = n 2 2 = l, x1 + x3 = 0, x1 + x2 = 0 = l,

\ x2 = l x3 = l x1 = −l

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11-18

Analytical Geometry

Similarly,

y1 = m  z1 = n



y2 = −m   z2 = n



y3 = m  

z3 = −n

Therefore, the vertices are (−l, m, n), (l, −m, n) and (l, m, −n). 1 1 Also, EF = BC ⇒ l 2 + m2 = a 2 2 ⇒ a 2 = 4( m2 + n2 )

b 2 = 4( n2 + l 2 )

and

c 2 = 4(l 2 + m2 ) A

F

B

E

D

C

\b 2 + c 2 − a 2 = 8l 2



c 2 + a 2 − b 2 = 8m 2



a 2 + b 2 − c 2 = 8n 2

Example 11.9 A directed line makes angles 60° and 60° with x- and y-axes, respectively. Find the angle it makes with z-axis.

Solution If a line makes angles a , b and g with x-, y- and z-axes, respectively then cos 2a + cos 2 b + cos 2g = 1. Here, a = 60°, b = 60°

\cos 2 60° + cos 2 60° + cos 2 g = 1

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Three Dimension

11-19

1 1 1 1 + + cos 2 g = 1 ⇒ cos 2 g = 1 − = 4 4 2 2 1 \cosg = ± or g = 45° or 135°. 2



Example 11.10 Find the acute angle between the lines whose direction ratios are 2, 1, −2 and 1, 1, 0.

Solution The direction cosines of the two lines are 2 1 −2 1 1 , , 0. , , and 3 3 3 2 2 If p is the angle between the lines then



cos θ = l1l2 + m1m2 + n1n2 =



2 3 2

+

1 3 2

=

1 2

\ p = 45°



Example 11.11 Find the angle between any two diagonals of a unit cube.

Solution A′(0, 1, 1)

C(0, 0, 1)

B′

O′ (1, 1, 1)

O (0, 0, 0) B (0, 1, 0)

A(1, 0, 0)

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C′(1, 1, 0)

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11-20

Analytical Geometry

The four diagonals of the cube are OO′, AA′, BB′ and CC′. Then the direction ratios of OO′ and AA′ are (1, 1, 1) and (−1, 1, 1). The direction cosines of OO′ and  −1 1 1   1 1 1  AA′ are  , , and  , , . If p is the angle between these  3 3 3   3 3 3  two diagonals then cos θ =

−1 1 1 1 + + = . 3 3 3 3

\q = cos −1  1   3

 1 Similarly, the angle between any two diagonals is cos −1   .  3

Example 11.12 If a , b , g and d are the angles made by a line with the four diagonals of a 4 cube, prove that cos 2 a + cos 2 b + cos 2 g + cos 2 d = . 3

Solution The four diagonals are OO′, AA′, BB′ and CC′ (refer figure given in Example 11.11). Let l, m, n be the direction cosines of the line making angles a , b , g and d with the four diagonals. Then, cosa =

3 l

+

m 3 m

+

n 3 n

, cos b = −

l 3 l

+

m 3 m

+

n 3 n

− + + − ,  cosd = 3 3 3 3 3 3 Squaring and adding these four results, we get cos 2 a + cos 2 b + 4 cos 2 g + cos 2 d = . 3

cosg =

l

Example 11.13 If l1 , m1 , n1 and l2 , m2 , n2 are the direction cosines of two mutually perpendicular lines, show that the direction cosines of the lines perpendicular to the above two lines are

m1n2 − m2 n1 , n1l2 − l1n2 and l1m2 − l2 m1.

Solution Let l, m and n be the direction cosines of the line perpendicular to the two given lines. Then ll1 + mm1 + nn1 = 0; ll2 , + mm2 + nn2 = 0

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Three Dimension



\

l m n = = = m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1 =



∑

q =

p p and sin = 1. 2 2

l 2 + m 2 + n2

∑ (m n

1 2

− m2 n1 )2

1

∑ (m n

1 2

But

11-21

− m2 n1 )2

( m1n2 − m2 n1 )2 = sin q and since the two lines are perpendicular

Therefore, the direction cosines of the line perpendicular to the given two lines are m1n2 − m2 n1 , n1l2 − n 2 l1 , l1m2 − l2 m1 .

Example 11.14 Show that three concurrent straight lines with direction cosines l1 , m1 , n1; l2 , m2 , n2 and l3 , m3 , n3 are coplanar if l1 m1 n1

l2 m2 n2 = 0 . l3 m3 n3

Solution Let l, m and n be the direction cosines of the line which is perpendicular to the given three lines. If the lines are coplanar then the line with direction cosines l, m and n is normal to the given coplanar line. ll1 + mm1 + nn1 = 0

ll2 + mm2 + nn2 = 0 ll3 + mm3 + nn3 = 0 l1 m1 n1

Eliminating l, m and n we get l2 m2 n2 = 0 . l3 m3 n3

Example 11.15 Prove that the straight lines whose direction cosines are given by the equations al + bm + cn = 0 and fmn + gnl + hlm = 0 are perpendicular if f g h + + = 0. a h c

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11-22

Analytical Geometry

Solution The direction cosines of two lines are given by al + bm + cn = 0  fmn + gnl + hlm = 0  −( al + bm) From (11.5), n = c Substituting in (11.6), we get

(11.5) (11.6)

 al + bm  −( fm + gl )   + hlm = 0  c  agl 2 + ( af + hg − ch)lm + bfm2 = 0

(i.e.)

Dividing by m2, we get 2

l  l ag   + ( af + hg − ch) + hf = 0  m m



(11.7)

If l1 , m1 , n1 and l2 , m2 , n2 are the direction cosines of the two given lines then l1 l ll hf and 2 are the roots of the equation (11.7), then 1 2 = . m1 m2 m1n2 ag Similarly,

m1m2 cg nn ah = and 1 2 = n1n2 bh l1l2 cf

But l1l2 + m1m2 + n1n2 = 0 Dividing n1n2 ,

l1l2 m1m2 + +1 = 0 n1n2 n1n2 cf cg + +1 = 0 ⇒ ah bh

f g h + + =0 a h c

Example 11.16 If two pair of opposite edges of a tetrahedron are at right angles then show that the third pair is also at right angles.

Solution

A

O B

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C

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Three Dimension

11-23

Let (OA, BC), (OB, CA) and (OC, AB) be three pair of opposite edges. Let O be the origin. Let the coordinates of A, B and C be ( x1 , y1 , z1 ), ( x2 , y2 , z2 ) and ( x3 , y3 , z3 ), respectively. Then the direction ratios of OA and BC are x1, y1, z1 and x2 − x3, y2 − y3, z2 − z3. Since OA is perpendicular to BC, we get x1 ( x2 − x3 ) + y1 ( y2 − y3 ) + z1 ( z2 − z3 ) = 0

(11.8)

Since OB is perpendicular to AC we get x2 ( x3 − x1 ) + y3 ( y3 − y1 ) + z2 ( z3 − z1 ) = 0

(11.9)



Adding (11.8) and (11.9), we get x3 ( x1 − x2 ) + y3 ( y2 − y1 ) + z3 ( z1 − z2 ) = 0



(11.10)

This shows that OC is perpendicular to AB.

Example 11.17 If l1 , m1 , n1; l2 , m2 , n2 and l3 , m3 , n3 be the direction cosines of the mutually perpendicular lines then show that the line whose direction ratios l1 + l2 + l3, m1 + m2 + m3 and n1 + n2 + n3 make equal angles with them.

Solution If l1 , m1 , n1; l2 , m2 , n2 and l3 , m3 , n3 are the direction cosines of three mutually perpendicular lines l12 + m12 + n12 = 1; l2 2 + m2 2 + n2 2 = 1;l32 + m32 + n32 = 1 also l1l2 + m1m2 + n1n2 = 0 l2 l3 + m2 m3 + n2 n3 = 0



l3 l1 + m3 m1 + n3 n1 = 0 Let q be the angle between the lines with the direction cosines l1 , m1 , n1 and direction ratios l1 + l2 + l3 , m1 + m2 + m3 and n1 + n2 + n3. Then,    cos q =

l1 (l1 + l2 + l3 ) + m1 ( m1 + m2 + m3 ) + n( n1 + n2 + n3 )

∑ (l

1



=

+ l2 + l3 )2

1

∑ (l

1

+ l2 + l3 )2

Similarly, the other two angles are equal to the same value of q. Therefore, the lines with the direction ratios l1 + l2 + l3 , m1 + m2 + m3 , n1 + n2 + n3 are equally inclined to the line with direction cosines l1 , m1 , n1; l2 , m2 , n2 and l3 , m3 , n3 .

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11-24

Analytical Geometry

Example 11.18 Show that the straight lines whose direction cosines are given by a 2 l + b 2 m + c 2 n = 0 , mn + nl + lm = 0 will be parallel if a ± b ± c = 0.

Solution Given the direction cosines of two given lines satisfy the equations

a2 l + b2 m + c2 n = 0

(11.11)



mn + nl + lm = 0

(11.12)

−( a 2 l + b 2 m) . c2 Substituting this value of n in (11.12), we get From (11.11), n =

−( a 2 l + b 2 m)( m + l ) + lm = 0 c2



a 2 l 2 + ( a 2 + b 2 + c 2 )lm + b 2 m2 = 0

(i.e.) Dividing by

m2,

a2 l 2 l + ( a2 + b2 − c 2 ) + b2 = 0 2 m m

(11.13)

If l1 , m1 , n1 and l2 , m2 , n2 are the direction cosines of the two given lines then l1 l2 , are the roots of the equation (11.13). m1 m2 l l Also, if the lines are parallel then 1 = 2 then the roots of the equation m1 m2 (11.13) are equal. The condition for that is the discriminant is equal to zero.

\ ( a2 + b2 − c 2 )2 − 4a2 b2 = 0



( a 2 + b 2 − c 2 ) = ±2ab ( a ± b) 2 = c 2 a±b = ±c



∴a ± b ± c = 0

Example 11.19 The projections of a line on the axes are 3, 4, 12. Find the length and direction cosines of the line.

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Three Dimension

11-25

Solution Let (l, m, n) be the direction cosines of the line and ( x1 , y1 , z1 ) and ( x2 , y2 , z2 ) be the extremities of the line. The direction cosines of x-, y and z-axes are (1, 0, 0), (0, 1, 0) and (0, 0, 1), respectively. The projection of the line on the axis is 3.

\ x2 − x1 = 3.

Similarly, y2 − y1 = 4, z2 − z1 = 12 Then the length of the line = ( x2 − x1 )2 + ( y2 − y1 )2 + ( z2 − z1 )2 = 32 + 4 2 + 122 = 13



Then direction ratios of the line are 3, 4, 12.  3 4 12  Therefore, the direction cosines of the line are  , ,  .  13 13 13 

Exercises 1.

Show that the points (10, 7, 0), (6, 6, −1) and (6, 9, −4) form an isosceles right-angled triangle.

2.

Show that the points (2, 3, 5), (7, 5, −1) and (4, −3, 2) form an isosceles triangle.

3.

Show that the points (1, 2, 3), (2, 3, 1) and (3, 1, 2) form an equilateral triangle.

4.

Show that the points (4, 0, 5), (2, 1, 3) and (1, 3, 2) are collinear.

5.

Show that the points (1, −1, 1), (5, −5, 4), (5, 0, 8) and (1, 4, 5) form a rhombus.

6.

Prove that the points (2, −1, 0), (0, −1, −1), (1, 1, −3) and (3, 1, −2) form the vertices of a rectangle.

7.

Show that the points (1, 2, 3), (−1, 2, −1), (2, 3, 2) and (4, 7, 6) form a parallelogram.

8.

Show that the points (−4, 3, 6), (−5, 2, 2), (−8, 5, 2), (−7, 6, 6) form a rhombus.

9.

Show that the points (4, −1, 2), (0, −2, 3), (1, −5, −1) and (2, 0, 1) lie on a sphere whose centre is (2, −3, 1) and find its radius.

10.

Find the ratio in which the line joining points (2, 4, 5) and (3, 5, −4) is divided by the xy-plane. Ans.: (5, 4)



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11-26

11.  12. 

Analytical Geometry

The line joining the points A(−2, 6, 4) and B(1, 3, 7) meets the YOZ-plane at C. Find the coordinates of C. Ans.: (0, 4, 6) Three vertices of a parallelogram ABCD are A(3, −4, 7), B(−5, 3, −2) and C(1, 2, −3). Find the coordinates of D. Ans.: (9, −5, 6)

13.

Show that the points (−5, 6, 8), (1, 8, 11), (4, 2, 9) and (−2, 0, 6) are the vertices of a square.

14.

Show that the points P(3, 2, −4), Q(9, 8, −10) and R(5, 4, −6) are collinear. Find the ratio in which R divides PQ. Ans.: (1, 2)



15. Find the ratio in which the coordinate planes divide the line joining the points (−2, 4, 7) and (3, −5, 8).  Ans.: 7:9; 4:5; −7: −8 16.

Prove that the line drawn from the vertices of a tetrahedron to the centroids of the opposite faces meet in a point which divides them in the ratio 3:1.

17.

Find the coordinate of the circumcentre of the triangle formed by the points with vertices (1, 2, 1), (−2, 2, −1) and (1, 1, 0). −1 Ans.:  , 2, 0  2 

 18.

A and B are the points (2, 3, 5) and (7, 2, 4). Find the coordinates of the points which the bisectors of the angles AOB meet AB.

19.

Find the length of the median through A of the triangle A(2, −1, 4), B(3, 7, −6) and C(−5, 0, 2). Ans.: 7

 20.

Prove that the locus of a point, the sum of whose distances from the points x2 y2 + z2 (a, 0, 0) and (−a, 0, 0) is a constant 2k, is the curve 2 + 2 =1. k k −a

21.

What are the direction cosines of the line which is equally inclined to the axes? 1 1   1 Ans.:  ± ,± ,±   3 3 3

 22.  23.

Find the angle between the lines whose direction ratios are (2, 3, 4) and (1, −2, 1). p Ans.: 2 A variable line in two adjacent positions has direction cosines (l, m, n), (l + d l , m + d m, n + d n). Prove that the small angle dq between two positives is given by d 2q = (d l )2 + (d m)2 + (d n)2.

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Three Dimension

24.  25. 26.  27.

11-27

Find the angle between the lines AB and CD, where A, B, C and D are the points (3, 4, 5), (4, 6, 3), (−1, 2, 4) and (1, 0, 5), respectively. 4 Ans.: cos −1    9 Prove by direction cosines the points (1, −2, 3), (2, 3, −4) and (0, −7, 10) are collinear. Find the angle between the lines whose direction ratios are (2, 1, −2) and (1, −1, 0). p Ans.: q = 4 Show that the line joining the points (1, 2, 3) and (1, 5, 7) is parallel to the line joining the points (−4, 3, −6) and (2, 9, 2).

28.

P, Q, R and S are the points (2, 3, −1), (3, 5, 3), (1, 2, 3) and (2, 5, 7). Show that PQ is perpendicular to RS.

29.

Prove that the three lines with direction ratios (1, −1, 1), (1, −3, 0) and (1, 0, 3) lie in a plane.

30.

Show that the lines whose direction cosines are given by al + bm + cn = 0 and al 2 + bm2 + cn2 = 0 are parallel if

31.

32.

af ± hg ± ch = 0 .

Show that the lines whose direction cosines are given by the equations al + vm + wn = 0 and al 2 + bm2 + cn2 = 0 are parallel if 2 2 u v w2 u 2 (b + c) + v 2 (c + a) + w 2 ( a + b) = 0 and perpendicular if + + =0. a b c If the edges of a rectangular parallelepiped are a, b and c, show that the angle  ± a2 ± b2 ± c2  between the four diagonals are given by cos −1  2 .  a + b 2 + c 2 

33.

If in a tetrahedron the sum of the squares of opposite edges is equal, show that its pairs of opposite sides are at right angles.

34.

Find the angle between the lines whose direction cosines are given by the equations:



(i) l + m + n = 0 and l 2 + m2 − n2 = 0 (ii) l + m + n = 0 and 2lm − 2nl − mn = 0



Ans.: (i) (ii)

35.

p 3 2p 3

If ( l1 , m1 , n1 ) and (l2 , m2 , n2) are the direction cosines of two lines inclined at an angle q, show that the actual direction cosines of the direction between the q m + m2 q n +n q  l1 + l2 sec , 1 sec , 1 2 sec  . lines are  2 2 2 2 2 2

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11-28

Analytical Geometry

36.

AB, BC are the diagonals of adjacent faces of a rectangular box with centre at the origin O its edges being parallel to axes. If the angles AOB, BOC and COA are q ,f and w , respectively then prove that cosq + cos f + cos w = −1.

37.

If the projections of a line on the axes are 2, 3, 6 then find the length of the line. Ans.: 7

 38.

The distance between the points P and Q and the lengths of the projections of PQ on the coordinate planes are d1 , d2 and d3 , show that 2d 2 = d12 + d2 2 + d32.

39.

Show that the three lines through the origin with direction ratios (1, −1, 7), (1, −1, 0) and (1, 0, 3) lie on a plane.

40.

Show that the angle between the lines whose direction cosines are given by p 1 1 1 l + m + n = 0 and fmn + gnl + hlm = 0 is if + + = 0 . 3 f g h

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Chapter

12

Plane

12.1  Introduction In three-dimensional coordinate geometry, first we define a plane and from a plane we define a straight line. In this chapter, we define a plane and obtain its equation in different forms. We also derive formula to find the perpendicular distance from a given point to a plane. Also, we find the ratio in which a plane divides the line joining two given points.

Definition 12.1.1:  A plane is defined to be a surface such that the line joining any two points wholly lies on the surface.

12.2  General Equation of a Plane Every first degree equation in x, y and z represents a plane. Consider the first degree equation in x, y and z as ax + by + cz + d = 0



(12.1)

where a, b, c and d are constants. Let P ( x1 , y1 , z1 ) and Q( x2 , y2 , z2 ) be two points on the locus of equation (12.1). Then the coordinates of the points that divide  x1 + l x2 y1 + l y2 z1 + l z2  , , . line joining these two points in the ratio l:1 are   l +1 l +1 1 + l  If this point lies on the locus of equation (12.1) then

a( x1 + l x2 ) b( y1 + l y2 ) c( z1 + l x2 ) + d = 0. + + l +1 l +1 l +1



(i.e.)  a( x1 + l x2 ) + b( y1 + l y2 ) + c( z1 + l z2 ) + d ( l + 1) = 0

(12.2)

Since P ( x1 , y1 , z1 ) and Q( x2 , y2 , z2 ) are two points on the locus of the equation (12.1) these two points have to satisfy the locus of the equation (12.1).

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12-2

Analytical Geometry

(i.e.)  ax1 + by1 + cz1 + d = 0

(12.3)

    ax2 + by2 + cz2 + d = 0

(12.4)



Multiplying (12.4) by l and adding with (12.3), we get ( ax1 + by1 + cz1 + d ) + l(ax2 + ( ax1 + by1 + cz1 + d ) + l(ax2 + by2 + cz2 + d ) = 0 which is the equation (12.2).  x1 + l x2 y1 + l y2 z1 + l x2  , , lies on the locus of Therefore, the point   l +1 l +1 1 + l  equation (12.1). Hence, this shows that if two points lies on the locus of equation (12.1) then every point on this line is also a point on the locus of equation (12.1). Hence, the equation (12.1) represents a plane and thus we have shown that every first degree equation in x, y and z represents a plane.

12.3  General Equation of a Plane Passing Through a Given Point Let the equation of the plane passing through a given point ( x1 , y1 , z1 ) be ax + by + cz + d = 0



(12.5)

since ( x1 , y1 , z1 ) lies on the plane (12.5). ax1 + by1 + cz1 + d = 0



(12.6)

Subtracting (12.6) from (12.5), we get a( x − x1 ) + b( y − y1 ) + c( z − z1 ) = 0. This is the general equation of the plane passing through the given point ( x1 , y1 , z1 ).

12.4  EQUATION OF A PLANE IN INTERCEPT FORM z

C O

x

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B

y

A

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Plane

12-3

Let the equation of a plane be Ax + By + Cz + D = 0



(12.7)

Let this plane make intercepts a, b and c on the axes of coordinates. If this plane meets the x-, y- and z-axes at A, B and C then their coordinates are (a, 0, 0), (0, b, 0) and (0, 0, c), respectively. Since these points lie on the plane Ax + By + Cz + D = 0, the coordinates of the points have to satisfy the ­equation Ax + By + Cz + D = 0.

∴ Aa + D = 0



Bb + D = 0

Cc + D = 0 −D −D −D ∴ A= ,B = ,C = a b c x y z By replacing the values of A, B and C, we get + + = 1. a b c This equation is called the intercept form of a plane.

12.5  Equation of a Plane in Normal Form z

C N O

P B

y

A x

Let a plane meet the coordinate axes at A, B and C. Draw ON perpendicular to the plane ABC and let ON = p. Let the direction cosines of ON be (cosa, cosb, cosg  ). Since ON = p, the coordinates of N are (pcosa, pcosb, pcosg  ). Let p( x1 , y1 , z1 ) be any point in the plane ABC. If a line is perpendicular to a plane then it is perpendicular to every line to the plane. Therefore, ON is perpendicular to OP. Since the coordinates of P and N are ( x1 , y1 , z1 ) and (pcosa, pcosb, pcosg  ) the direction ratios of N are ( x1 − p cos a , y1 − p cos b , z1 − p cosg ) since N is perpendicular to ON.

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12-4



Analytical Geometry

cos a ( x1 − p cos a ) + cos b ( y1 − p cos b ) + cos g ( z1 − p cos g ) = 0

(i.e.) x1 cos a + y1 cos b + z1 + cos g = p(cos 2 a + cos 2 b + cos 2 g ) = p cos 2 a + cos 2 b + cos 2 g = 1

since

Therefore, the locus of ( x1 , y1 , z1 ) is xcosa + ycosb + zcosg  = p. This equation is called the normal form of a plane.

Note 12.5.1: Here, the coefficients of x, y and z are the direction cosines of normal to the plane and p is the perpendicular distance from the origin on the plane.

Note 12.5.2: Reduction of a plane to normal form: the equation of plane in general form is Ax + By + Cz + D = 0



(12.8)

Its equation in normal form is lx + my + nz = p A B C D Identifying (12.8) and (12.9), we get = = = l m n p







(12.9)

l m n ± l 2 + m 2 + n2 ±1 . = = = = 2 2 2 2 A B C A + B +C A + B2 + C 2 ±A ±B l= ,m = , 2 2 2 2 A + B +C A + B2 + C 2 ∴ n=

±C

,p=

±D

A + B +C A + B2 + C 2 Since p has to be positive when D is positive, we have p=

m=

When D is negative, l = n=

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2

2

2

D A + B +C −B 2

2

2

A2 + B 2 + C 2 −A l +m +n −C 2

2

2

,l = ,n =

,m =

2

−A A + B2 + C 2 −C 2

,

A2 + B 2 + C 2 −B l + m 2 + n2 2

,

l 2 + m 2 + n2

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Plane

12-5

12.6  Angle Between Two Planes Let the equation of two planes be

a1 x + b1 y + c1 z + d = 0 a2 x + b2 y + c2 z + d2 = 0

(12.10) (12.11)

The direction ratios of the normals to the above planes are a1 b1 c1 a2 ± ,± ,± and ± , 2 2 2 2 2 2 2 2 2 2 a1 + b1 + c1 a1 + b1 + c1 a1 + b1 + c1 a2 + b22 + c22 b2 c2 ± ,± . a22 + b22 + c22 a22 + b22 + c22 The angle between two planes is defined to be the angle between the normals to the two planes. Let q be the angle between the planes. Then, cos q = l1l2 + m1m2 + n1n2 = ±



( a1a2 + b1b2 + c1c2 ) a + b12 + c12 a22 + b22 + c22 2 1

 ( a1a2 + b1b2 + c1c2 ) ∴q = cos −1  ± 2  a1 + b12 + c12 a22 + b22 + c22

  

Note 12.6.1: The positive sign of cosq gives the acute angle between the planes and negative sign gives the obtuse angle between the planes.

Note 12.6.2: If the planes are perpendicular then q = 90°.

∴ a1a2 + b1b2 + c1c2 = 0

Note 12.6.3: If the planes are parallel then direction cosines of the normals are proportional. a b c ∴ 1 = 1 = 1 a2 b2 c2 Note 12.6.4: The equation of plane parallel to ax + by + cz + d = 0 can be expressed in the form ax + by + cz + k = 0.

12.7  Perpendicular Distance From a Point on a Plane Let the equation of the plane be ax + by + cz + d = 0 (12.12) and P ( x1 , y1 , z1 ) be the given point. We have to find the perpendicular distance from P to the plane. The normal form of the plane (12.12) is

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12-6

Analytical Geometry

lx + my + nz = p (12.13) Draw PM perpendicular from P to the plane (12.12). Draw the plane through P to the given plane (12.12). The equation of this plane is lx + my + nz = p1     



(12.14)

where p1 is the perpendicular distance from the origin to the plane (12.13). This plane passes through ( x1 , y1 , z1 ). ∴ lx1 + my1 + nz1 = p1     



Draw BN perpendicular to the plane (12.3) meeting the plane (12.12) at M. Then ON = p1 and OM = p.

MN = OM − ON = p − p1 = p − (lx1 + my1 + nz1 ).

Comparing equations (12.12) and (12.13),



l m n − p ± l 2 + m 2 + n2 = = = = a b c d a2 + b2 + c 2 a b ∴l = ± ,m = ± , 2 2 2 2 a +b +c a + b2 + c2 c d n=± ,p=∓ 2 2 2 2 a +b +c a + b2 + c2

The perpendicular distance = p − (lx1 + my1 + nz1 )     ax + by1 + cz1 + d =± 1 a2 + b2 + c 2 ax + by1 + cz1 + d Therefore, the perpendicular distance from ( x1 , y1 , z1 ) is = 1 . a2 + b2 + c 2

Aliter:

z

C P (x1, y1, z1) Q O

M B

y

A x

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12-7

Plane

Let PM be the perpendicular from P on the plane ax + by + cz + d = 0. Let P ( x1 , y1 , z1 ) and M ( x2 , y2 , z2 ) be a point on the plane (12.12). Then QM and PM are perpendicular. Let q  be the MPQ . The direction ratios of PM and PN are (a, b, c) and (x1 - x2, y1-y2, z1 - z2). a( x1 − x2 ) + b( y1 − y2 ) + c( z1 − z2 )

Then, cosq =

a2 + b2 + c 2

Also PM = PQcosq =



∑ (x

1

− x2 ) 2

± a( x1 − x2 ) + b( y1 − y2 ) + c( z1 − z2 )

∑ a ∑ (x 2

1



=±



=±

− x2 ) 2

a( x1 − x2 ) + b( y1 − y2 ) + c( z1 − z2 ) a2 + b2 + c 2 ( ax1 + by1 + cz1 ) − ( ax2 + by2 + cz2 ) a +b +c 2

2

2

=±

ax1 + by1 + cz1 + d a2 + b2 + c 2

Note 12.7.1: The perpendicular distance from the origin to the plane ax + by + cz + d = 0 is

d a + b2 + c 2 2

.

12.8  Plane Passing Through Three Given Points Let ( x1 , y1 , z1 ),( x2 , y2 , z2 ) and ( x3 , y3 , z3 ) be three given points on a plane. Let the equation of the plane be ax + by + cz + d = 0 (12.15) Any plane through ( x1 , y1 , z1 ) is a( x1 − x2 ) + b( y1 − y2 ) + c( z1 − z2 ) = 0



(12.16)

This plane also passes through ( x2 , y2 , z2 ) and ( x3 , y3 , z3 ).

a( x2 − x1 ) + b( y2 − y1 ) + c( z2 − z1 ) = 0

(12.17)



a( x3 − x4 ) + b( y3 − y4 ) + c( z3 − z4 ) = 0

(12.18)

Eliminating a, b and c from (12.6), (12.7) and (12.18), we get x − x1 y − y1 z − z1 x2 − x1

y2 − y1

z2 − z1 = 0.

x3 − x1

y3 − y1

z3 − z1

This is the equation of the required plane.

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12-8

Analytical Geometry

Aliter: Let the equation of the plane be ax + by + cz + d = 0



(12.19)

This plane passes through the points ( x1 , y1 , z1 ),( x2 , y2 , z2 ) and ( x3 , y3 , z3 ). ax1 + by1 + cz1 + d = 0 ax2 + by2 + cz2 + d = 0 ax3 + by3 + cz3 + d = 0



(12.20) (12.21) (12.22)

Eliminating a, b, and c from (12.20), (12.21) and (12.22), we get x x1 x2 x3

y y1 y2 y3

z z1 z2 z3

1 1 = 0. 1 1

This is the equation of the required plane.

12.9  To Find the ratio in which the plane joining the points (x1, y1, z1) and (x2, y2, z2) is divided by the plane ax + by + cz + d = 0. P (x1, y1, z1)

R

Q (x2, y2, z2)

The equation of the plane is

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ax + by + cz + d = 0

(12.23)

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Plane

12-9

Let the line joining the points P ( x1 , y1 , z1 ) and Q( x2 , y2 , z2 ) meet the plane at R. Let R divided PQ in the ratio l :1. Then the coordinates of R are  x1 + l x2 y1 + l y2 z1 + l z2   1 + l , 1 + l , 1 + l  . This point lies on the plane given by (1). ∴

a( x1 + l x2 ) b( y1 + l y2 ) c( z1 + l z2 ) , , +d =0 1+ l 1+ l 1+ l (i.e.)  a( x1 + l x2 ) + b( y1 + l y2 ) + c( z1 + l z2 ) + d (1 + l ) = 0 (i.e.)  ax1 + by1 + cz1 + d + l( ax2 + by2 + cz2 + d ) = 0



∴l = −

ax1 + by1 + cz1 + d ax2 + by2 + cz2 + d

Note 12.8.1: If ( ax1 + by1 + cz1 + d ) and ( ax2 + by2 + cz2 + d ) are of the same sign then l is negative. Then the point R divides PQ externally and so the points P and Q lie on the same side of the plane. Note 12.8.2: If P ( x1 , y1 , z1 ) and the origin lie on the same side of the plane ax + by + cz + d = 0 if ax1 + by1 + cz1 + d and d of the same sign.

12.10  Plane passing through the intersection of two given planes Let the two given planes be

ax1 + by1 + cz1 + d1 = 0

(12.24)



ax2 + by2 + cz2 + d2 = 0

(12.25)

Then consider the equation ( ax1 + by1 + cz1 + d1 ) + l ( ax2 + by2 + cz2 + d2 ) = 0. This being the first degree equation in x, y and z, represents a plane. Let ( x1 , y1 , z1 ) be the point on the line of the intersection of planes given by ­equations (12.24) and (12.25). Then ( x1 , y1 , z1 ) lies on the two given planes.

∴ a1 x1 + b1 y1 + c1 z1 + d1 = 0

(12.26)



a2 x1 + b2 y1 + c2 z1 + d2 = 0

(12.27)

Then, clearly ( a1 x1 + b1 y1 + c1 z1 + d1 ) + l ( a2 x1 + b2 y1 + c2 z1 + d2 ) = 0.

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12-10

Analytical Geometry

From this equation, we infer that the point (x1, y1, z1) lies on the plane given by (12.26). Similarly, every point in the line of intersection of the planes (12.24) and (12.25) lie on the planes (12.24) and (12.25). Hence, equation (12.26) is the plane passing through the intersection of the two given planes.

12.11  Equation of the planes which bisect the angle between two given planes Find the equation of the planes which bisect the angle between two given planes. Let the two given planes be  a1 x + b1 y + c1 z + d1 = 0

(12.28)

a2 x + b2 y + c2 z + d2 = 0

(12.29)



Let P ( x1 , y1 , z1 ) be a point on either of the bisectors of the angle between the two given planes. Then the perpendicular distance from P to the two given planes are equal in magnitude.

(i.e.) 

a1 x1 + b1 y1 + c1 z1 + d1 a +b +c 2 1

The locus of ( x1 , y1 , z1 ) is

2 1

2 1

=±

a2 x1 + b2 y1 + c2 z1 + d2

a1 x + b1 y + c1 z + d1 a +b +c 2 1

2 1

2 1

a22 + b22 + c22 =±

a2 x + b2 y + c2 z + d2 a22 + b22 + c22

.

By taking the positive sign, we get the equation of one of the bisectors and by taking the negative sign, we get the equation to the other bisector.

Note 12.11.1:  We can determine which of the two planes bisects the acute angle between the planes. For this, we have to find the angle q between the bisector planes and one of the two given planes. If tanq < 1 (q < 45°), then the bisector plane taken is the internal bisector and the other bisector plane is the external bisector. If tanq  > 1 then the bisector plane taken is the external bisector and the other bisector plane is the internal bisector. Note 12.11.2:  We can also determine the equation of the plane bisecting the angle between the planes that contain the origin. Suppose the equation of the two planes are a1 x + b1 y + c1 z + d1 = 0 and a2 x + b2 y + c2 z + d2 = 0 where d1 and d2 are positive. Let P ( x1 , y1 , z1 ) be a point on the bisector between the angles of the planes containing the origin. Then d1 and

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Plane

12-11

a1 x + b1 y + c1 z + d are of the sign. Since d1 is positive, a1x + b1y + c1z is also positive. Similarly, a2 x + b2 y + c2 z + d2 is also positive. Therefore, the equation of the plane bisecting the angle containing the origin is a1 x + b1 y + c1 z + d1 a x + b y + c z + d2 =+ 2 1 2 1 2 1 . 2 2 2 a1 + b1 + c1 a22 + b22 + c22 The equation of bisector plane not containing the origin is a1 x + b1 y + c1 z + d1 a +b +c 2 1

2 1

2 1

=−

a2 x1 + b2 y + c2 z + d2 a22 + b22 + c22

.

12.12  Condition for the homogenous equation   of the second degree To represent a pair of planeS The equation that represents a pair of planes be ax 2 + by 2 + cz 2 + 2 fyz + 2 gzx + 2hxy = 0 . Let the two planes represented by the above homogenous equation of the second degree in x, y and z be lx + my + nz = 0 and l1 x + m1 y + n1 z = 0. Then ax 2 + by 2 + cz 2 + 2 fyz + 2 gzx + 2hxy = (lx + my + nz )(l1 x + m1 y + n1 z ). Comparing the like terms on both sides, we get ll1 = a, mm1 = b, nn1 = c \lm1 + l1m = 2h



 ln1 + l1n = 2 g mn1 + m1n = 2 f



l l1 Now, consider the result m m1 n n1



0 l1 0 × m1 0 n1

l 0 m 0 =0 n 0

ll1 + l1l lm1 + l1m ln1 + l1n (i.e.)  lm1 + l1m mm1 + m1m mn1 + m1n = 0 ln1 + l1n mm1 + m1m nn1 + n1n (i.e.) 

2a 2h 2h 2b 2g 2 f

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2g 2 f = 0  (i.e.)  2c

a h g

h b f

g f =0 c

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12-12

Analytical Geometry

(i.e.)  a(bc - g2) - h(ch - fg) + g(hf - hg) = 0 abc + 2fgh - af  2 - bg2 - ch2 = 0



This is the required condition.

Note 12.12.1: To find the angle between the two planes:

tan q =

=±

tan q = ±

sin q . cos q (l 2 + m2 + n2 )(l12 + m12 + n12 ) − (ll1 + mm1 + nn1 )2

Let q  be the angle between the two planes. Then tan q = 1 − cos 2 q =± cos q

ll1 + mm1 + nn1

( mn1 − m1n)2 + ( nl1 − n1l )2 + (lm1 − l1m)2 ll1 + mm1 + nn1

∑ (mn

− m1n)2

∑4 f

− 4bc

1

ll1 + mm1 + nn1 2

=±

∑ (mn + m n) 1

1

2

− 4 mm1nn1

a+b+c

2 f 2 + g 2 + h2 − bc − ca − ab a+b+c a+b+c  ±2 f 2 + g 2 + h2 − bc − ca − ab  q = tan −1   a+b+c   =±

=±

Note 12.12.2: If the planes are perpendicular then q = 90° and the condition for that is a + b + c = 0.

ILLUSTRATIVE EXAMPLES Example 12.1 The foot of the perpendicular from the origin to a plane is (13, -4, -3). Find the equation of the plane.

Solution The line joining the points (0, 0, 0) and (13, -4, -3) is normal to the plane. Therefore, the direction ratios of the normal to the plane are (13, -4, -3). The equation of the plane is a( x − x1 ) + b( y − y1 ) + c( z − z1 ) = 0 (i.e.) 

13( x − x1 ) − 4( y − y1 ) − 3( z − z1 ) = 0 13( x − 13) − 4( y + 4) − 3( z + 3) = 0 13 x − 4 y − 3 z − 169 − 16 − 9 = 0 13 x − 4 y − 3 z − 194 = 0

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Plane

12-13

Example 12.2 A plane meets the coordinate axes at A, B and C such that the centroid of the triangle is the point (a, b, c). Find the equation of the plane.

Solution Let the equation of the plane be

x y z + + = 1. Then the coordinates of A, B a b g

and C are (a, 0, 0), (0, b, 0) and (0, 0, g  ). The centroid of the triangle ABC is a b g   , ,  . But the centroid is given as (a, b, c). 3 3 3 a b g a = ,b = ,c = or a = 3a, b = 3b, g   = 3c 3 3 3 x y z x y z Therefore, the equation of the plane is + + = 1 or + + = 3. 3a 3b 3c a b c

Example 12.3 Find the equation of the plane passing through the points (2, 2, 1), (2, 3, 2) and (-1, 3, 1).

Solution The equation of the plane passing through the point (2, 2, 1) is of the form a(x - 2) + b(y - 2) + c(z - 1) = 0. This plane passes through the points (2, 3, 2) and (-1, 3, 1). ∴ 0a + b + c = 0 and -3a + b + 0c = 0 Solving we get

a b c a b c = = or = = 0 − 1 −3 − 0 0 + 3 1 3 −3

Therefore, the equation of the plane is 1(x - 2) + 3(y - 2) - 3(z - 1) = 0. \ x + 3y - 3z - 5 = 0

Example 12.4 Find the equation of the plane passing through the point (2, -3, 4) and parallel to the plane 2x - 5y - 7z + 15 = 0.

Solution The equation of the plane parallel to 2x - 5y - 7z + 15 = 0 is 2x - 5y - 7z + k = 0. This plane passes through the point (2, -3, 4).

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12-14

Analytical Geometry

∴ 4 + 15 - 28 + k = 0 or k = 9



Hence, the equation of the required plane is 2x - 5y - 7z + 9 = 0.

Example 12.5 Find the equation of the plane passing through the point (2, 2, 4) and perpendicular to the planes 2x - 2y - 4z - 3 = 0 and 3x + y + 6z - 4 = 0.

Solution Any plane passing through (2, 2, 4) is a(x - 2) + b(y - 2) + c(z - 4) = 0. This plane is perpendicular to the planes 2x - 2y - 4z - 3 = 0 and 3x + y + 6z - 4 = 0. (i.e.)  2a - 2b - 4c = 0 3a + b + 6c = 0



∴

a b c a b c or = = = = −12 + 4 −12 − 12 2 + 6 −8 −24 +8

Therefore, the direction ratios of the normal to the required plane are 1, 3, -1. Therefore, the equation of the plane is (x - 2) + 0 + (z - 4) = 0 (i.e.) (x - 2) + 3(y - 2) - (z - 4) = 0. x + 3y - z - 4 = 0



Example 12.6 Determine the constants k so that the planes x - 2y + kz = 0 and 2x + 5y - z = 0 are at right angles and in that case find the plane through the point (1, -1, -1) and perpendicular to both the given planes.

Solution The planes x - 2y + kz = 0 and 2x + 5y - z = 0 are perpendicular. Therefore, 2 - 10 - k = 0 ∴ k = -8. Any plane passing through (1, -1, -1) is a (x - 1) + b(y + 1) + c(x + 1) = 0. This plane is perpendicular to the planes x - 2y - 8z = 0 and 2x + 5y - z = 0.

∴ a - 2b - 8c = 0 and 2a + 5b - c = 0



a b c a b c = = = or = 2 + 40 −16 + 1 5 + 4 42 −15 9



⇒

a b c = = 14 −5 3

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Plane

12-15

Therefore, the equation of the required plane is 14(x - 1) - 5(y + 1) + 3(z + 1) = 0 or 14x - 5y + 3z - 16 = 0.

Example 12.7 A variable plane is at a constant distance p from the origin and meets the axes in A, B and C. Show that the locus of the centroid of the tetrahedron OABC is x-2 + y-2 + z-2 = 16p-2.

Solution z C

x

A

B

y

x y z + + = 1. Then the coordinates of a b c O, A, B and C are (0, 0, 0), (a, 0, 0), (0, b, 0) and (0, 0, c). Let the centroid of the tetrahedron OABC be ( x1 , y1 , z1 ). But the centroid of  a b c the tetrahedron is  , ,   4 4 4 a b c ∴ = x1 , = y1 , = z1 4 4 4 ∴ a = 4 x1 , a = 4 y1 , a = 4 z1 Let the equation of the plane ABC be

The perpendicular distance from O and the plane ABC is p. 1 1 1 1 1 = p or 2 + 2 + 2 = 2 a b c p 1 1 1 + 2+ 2 2 a b c 1 1 1 1 ∴ + + = or x1−2 + y1−2 + z1−2 = 16 p −2 16 x12 16 y12 16 z12 p 2 The locus of ( x1 , y1 , z1 ) is x −2 + y −2 + z −2 = 16 p −2 .

Example 12.8 Two systems of rectangular axes have the same origin. If a plane cuts them at distances (a, b, c) and (a1 , b1 , c1 ) respectively, from the origin, prove that 1 1 1 1 1 1 + 2 + 2 = 2 + 2 + 2. 2 a b c a1 b1 c1

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12-16

Analytical Geometry

Solution Let (o, x, y, z) and (O, X, Y, Z) be the two system of coordinate axes. The equation of the plane with respective first system of coordinate axis is x y z + + = 1 (12.30) a b c The equation of the same plane with respect to the second system of coordinate axes is X Y Z + + = 1 (12.31) a1 b1 c1 The perpendicular distance from the origin to the plane given by the equation (12.30) is 1 (12.32) 1 1 1 + + a1 b1 c1



The perpendicular distance from the origin to the plane is given by the equa1 tion (12.31) is . 1 1 1 + + a12 b12 c12 Since the equations (12.30) and (12.31) represent the same plane these two perpendicular distances are equal. 1 1 (i.e.) = 1 1 1 1 1 1 + + + 2+ 2 2 a1 b1 c1 a1 b1 c1 or

1 1 1 1 1 1 + 2+ 2 = 2+ 2+ 2 2 a b c a1 b1 c1

Example 12.9 A variable plane passes through a fixed point (a, b, c) and meets the coordinate axes in A, B and C. Prove that the locus of the point of intersection of a b c planes through A, B and C parallel to the coordinate planes is + + = 1. x y z

Solution

x y z + + = 1. a b g This plane passes through the point (a, b, c). Let the equation of the plane be

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Plane

12-17

a b c ∴ + + = 1 (12.33) a b g Then the equation of the planes through A, B and C parallel to the coordinate planes are x = a, y = b and z = g. Let ( x1 , y1 , z1 ) be the point of intersection of these planes. Then x1 = a , y1 = b and z1 = g a b c Therefore, from equation (12.33), we get + + = 1. The locus of x y1 z1 1 a b c ( x1 , y1 , z1 ) is + + = 1. x y z

Example 12.10 A variable plane makes intercepts on the coordinate axes, the sum of whose squares is constant and is equal to k2. Prove that the locus of the foot of the perpendicular from the origin to the plane is (x2 + y2 + z2) (x-2 + y-2 + z-2) = k2.

Solution Let the equation of the plane be x y z + + =1 (12.34) a b c where a, b and c are the intercepts on the coordinate axes. Given that a2 + b2 + c2 = k2 (12.35) Let P ( x1 , y1 , z1 ) be the foot of the perpendicular from O on this plane. The direction ratios of the normal OP are 1 , 1 , 1 . Therefore, the equaa b c tion of the normal OP are ax = by = cz. Since ( x1 , y1 , z1 ) lies on the normal, ax1 = by1 + cz1 = t (say). t t t ∴a = , b = , c = (12.36) x1 y1 z1 From (12.34) and (12.35), we get  1 1 1 (12.37) t 2  2 + 2 + 2  = k 2 y z  x 1

1

1

The point ( x1 , y1 , z1 ) also lies on the plane. x y z ∴ 1 + 1 + 1 = 1 a b c Eliminating a, b, c from (12.36) and (12.38) 1 2 ( x1 + y12 + z12 ) = 1 t

M12_ANALYTICALGEOMETRY_ISBN_CH12.indd 17

(12.38)

(12.39)

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12-18

Analytical Geometry

Eliminating t from (12.38) and (12.39) ( x12 + y12 + z12 )( x1−2 + y1−2 + z1−2 ) = k 2 Therefore, the locus of ( x1 , y1 , z1 ) is ( x 2 + y 2 + z 2 )( x −2 + y −2 + z −2 ) = k 2 .

Example 12.11 Find the equation of the plane which cuts the coordinate axes at A, B, and C such that the centroid of ∆ ABC is at the point (-1, -2, -4).

Solution

x y z + + = 1. Then the coordinates of A, B a b c  a b c and C are (a, 0, 0), (0, b, 0), (0, 0, c). The centroid of ∆ ABC is  , ,  . But  3 3 3 a b c the centroid is given as (-1, -2, -4). ∴ = −1, = −2, = −4 3 3 3 ∴ a = -3, b = -6, c = -12 x y z + + = 1. Hence the equation of the plane ABC is −3 −6 −12 (i.e.)  4x + 2y + z + 12 = 0 Let the equation of the plane be

Example 12.12 Find the equation of the plane passing through the point (-1, 3, 2) and perpendicular to the planes x + 2y + 2z = 5 and 3x + 3y + 2z = 8.

Solution The equation of the plane passing through the point (-1, 3, 2) is A(x + 1) + B(y - 3) + C(z - 2) = 0. This plane is perpendicular to the planes x + 2y + 2z = 5 and 3x + 3y + 2z = 8. If two planes are perpendicular then their normals are perpendicular. The direction ratios of the normal to the required plane are A, B and C. The direction ratios of the normals to the given planes are 1, 2, 2 and 3, 3, 2. A + 2B + 2C = 0 3A + 3B + 2C = 0 A B C A B C = = or = = 4 −6 6−2 3−6 −2 4 −3 Therefore, the direction ratios of the normal to the required plane are 2, -4, 3. The equation of the required plane is 2(x + 1) - 4(y - 3) + 3(z - 2) = 0 (i.e.) 2x - 4y + 3z + 8 = 0.

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Plane

12-19

Example 12.13 Find the equation of the plane passing through the points (9, 3, 6) and (2, 2, 1) and perpendicular to the plane 2x + 6y + 6z - 9 + 0.

Solution Any plane passing through the point (9, 3, 6) is A(x - 9) + B( y - 3) + C(z - 6) = 0



(12.40)

The plane also passes through the point (2, 2, 1). ∴  → -7A - B - 5C = 0 or 7A + B + 5C = 0



(12.41)

The plane (12.40) is perpendicular to the plane. 2x + 6y + 6z - 9 = 0 ∴  2A + 6B + 6C = 0 (i.e.)  7A + B + 5C = 0 2A + 6B + 6C = 0



(12.42) (12.43)

A B C A B C A B C = = ⇒ = = or = = 6 − 30 10 − 42 42 − 2 −24 −32 40 3 4 −5

Therefore, the equation of the required plane is 3(x - 9) + 4(y - 3) -  5(z - 6) = 0.

\  3x + 4y - 5z = 9

Example 12.14 Show that the following points (0, -1, 0), (2, 1, -1), (1, 1, 1) and (3, 3, 0) are coplanar.

Solution The equation of the plane passing through the point (0, -1, 0) is Ax + B(y + 1) + Cz = 0. This plane passes through the points (2, 1, -1) and (1, 1, 1).

∴  2A + 2B - C = 0



A + 2B + C = 0



M12_ANALYTICALGEOMETRY_ISBN_CH12.indd 19

∴ 

A B C = = 4 −3 2

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12-20

Analytical Geometry

Therefore, the equation of the plane is 4x - 3(y + 1) + 2z = 0.

\  4x - 3y + 2z - 3 = 0

Substituting x = 3, y = 3, z = 0 we get 12 - 9 - 3 = 0 which is true. Therefore, the plane passes through the points (3, 3, 0) and hence the four given points are coplanar.

Example 12.15 Find for what values of l, the points (0, -1, l), (4, 5, 1), (3, 9, 4) and (-4, 4, 4) are coplanar.

Solution The equation of the plane passing through the point (4, 5, 1) is A(x - 4) + B(y - 5) + C(z - 1) = 0. This plane passes through the points (3, 9, 4) and (-4, 4, 4).

∴  -A + 4B + 3C = 0



-8A - B + 3C = 0



A B C A B C = = = or = 15 −21 33 5 −7 11 Therefore, the equation of plane is 5(x - 4) - 7(y - 5) + 11(z - 1) = 0. (i.e.)  5x - 7y + 11z + 4 = 0



If this plane passes through the point (0, -1, l) then 0 + 7 + 11l + 4 = 0. ∴ l = -1

Example 12.16 A variable plane moves in such a way that the sum of the reciprocals of the intercepts on the coordinate axes is a constant. Show that the plane passes through a fixed point.

Solution Let the equation of the plane be x y z + + = 1 (12.44) a b c Given that the sum of the reciprocals of the intercepts is a constant. 1 1 1 + + =k ∴  a b c



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Plane



1 1 1 + + = 1 ak bk ck

(i.e.)  (12.44) - (12.45) gives

12-21

(12.45)

1 1 1  1 1  1  x −  +  y −  +  z −  = 0 a k b k c k

 1 1 1 This planes passes through the fixed point  , ,  . k k k

Example 12.17

x y z + + = 1 and the plane through P pera b c pendicular to OP meets the axes in A, B and C. If the planes through A, B and C are parallel to the coordinates planes meet in a point then show that the 1 1 1 1 1 1 + + . locus of Q is 2 + 2 + 2 = ax by cz x y z

A point P moves on fixed plane

Solution The given plane is x y z + + = 1 a b c



(12.46)

Let P be the point (a, b, g  ). The plane passes through P. ∴ 



a b g + + = 1 a b c

(12.47)

The equation of the plane normal to OP is

a (x - a) + b (y - b ) + g (z - g  ) = 0 (i.e.) a x + by + g z = a 2 + b 2 + g 2

(12.48)

The intercepts made by this plane on the coordinate axes are a 2 + b 2 + g 2     a 2 + b 2 + g 2      a 2 + b 2 + g 2      ,y= ,z = . x= a b g If these planes meet at ( x1 , y1 , z1 ) then a 2 + b 2 + g 2     a 2 + b 2 + g 2     a 2 + b 2 + g 2      , y1 = , z1 =   (12.49) a b g Now we have to eliminate a, b, g   using (12.47) and (12.49).

x1 =

From (12.49),

1 1 1 1 + + = x12 y12 z12 a 2 + b 2 + g 2  

M12_ANALYTICALGEOMETRY_ISBN_CH12.indd 21

(12.50)

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12-22

Analytical Geometry

 1 1 1  + +  = 1 or From (12.47) and (12.49), ∑ a 2   ax1 by1 cz1  1 1 1 1 + + = 2 ax1 by1 cz1 a + b 2 + g 2  



(12.51)

1 1 1 1 1 1 + + from Therefore, the locus of ( x1 , y1 , z1 ) is 2 + 2 + 2 = ax by cz x y z (12.50) and (12.51).

Example 12.18 If from the point P(a, b, c) perpendiculars PL, PM be drawn to YZ- and ZXplanes, find the equation of the plane OLM.

Solution P is the point (a, b, c). PL is drawn perpendicular to YZ-plane. Therefore, the coordinates of L are (0, b, 0). PM is drawn perpendicular to ZX-plane. Therefore, the coordinates of M are (0, 0, c). We have to find the equation of the plane OLM. The equation of the plane passing through (0, 0, 0) is Ax + By + Cz = 0.This plane also passes through (0, b, c), (a, 0, c). ∴  O ⋅ A + Bb + Cc = 0 Aa + O ⋅ B + Cc = 0



∴ 



A B C = = bc ca − ab

The equation of the plane OLM is bcx + cay - abz = 0.

Example 12.19  −1  Show that  , 2, 0 is the circumcentre of the triangle formed by the points  2  (1, 1, 0), (1, 2, 1) and (-2, 2, -1).

Solution A, B and C are the points (1, 1, 0), (1, 2, 1) and (-2, 2, -1) and P is the point  −1   , 2, 0 . To prove that P is the circumcentre of the triangle ABC, we have 2 to show that: (i) the points P, A, B and C are coplanar and (ii) PA = PB = PC.

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Plane

12-23

The equation of the plane through the point (1, 1, 0) is A(x - 1) + B(y - 1) + C(z - 0) = 0. This plane also passes through (1, 2, 1) and (-2, 2, -1).

OA + B + C = 0



-3A + B - C = 0



A B C = = −2 −3 3 Therefore, the equation of the plane ABC is -2(x - 1) - 3(y - 1) + 3z = 0. (i.e.)  2x + 3y - 3z - 5 = 0



 −1  Substituting the coordinates of P  , 2, 0 we get -1 + 6 - 0 - 5 = 0  2  which is true. Therefore, the points P, A, B and C are coplanar. Now 2

13  1  PA =  − − 1 + (2 − 1)2 + ( 0 − 0) 2 =  2  4 2

2

13  1  PB =  − − 1 + (2 − 2)2 + (0 − 1)2 =  2  4 2



2

13  1  PC =  − + 2 + (2 − 2)2 + (0 − 1)2 =  2  4 2

∴ PA = PB = PC =

13 . 4

Therefore, P is the circumcentre of the triangle ABC.

Example 12.20 Find the ratio in which the line joining the points (2, -1, 4) and (6, 2, 4) is divided by the plane x + 2y + 3z + 5 = 0.

Solution Let the plane x + 2y + 3z + 5 = 0 divide the line joining the points P (2, -1, 4) and Q (6, 2, 4) in the ratio l : 1.  6l + 2 2l − 1 4 l + 4  Then the point of division is  , ,  l + 1 l + 1 l + 1 

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12-24

Analytical Geometry

This point lies on the plane x + 2y + 3z + 5 = 0. ∴ 



6l + 2  2l − 1  4l + 4  + 2  + 3   +5= 0  l +1 l +1 l +1 



(i.e.)  (6l + 2) + 2(2l − 1) + 3( 4 l + 4) + 5( l + 1) = 0



6l + 2 + 4 l − 2 + 12l + 12 + 5l + 5 = 0 27l + 17 = 0 or l =



−17 27

Therefore, the plane divides the line externally in the ratio 17:27.

Example 12.21 A plane triangle whose sides are of length a, b, and c is placed so that the middle points of the sides are on the axes. If a, b and g   are intercepts on x y z the axes then show that the equation of the plane is + + = 1 where a b g 1 1 1 a2 + b2 − c 2 . a = b2 + c2 − a2 , b = c 2 + a 2 − b 2 , and g = 2 2 2 2 2 2

Solution z B

N

A

g a x

O b

M

y

L C

x y z + + = 1. a b g Let the plane meet the axes at L, M, N respectively. L (a, 0, 0), M (0, b, 0), N (0, 0, g) The equation of the plane is



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LM 2 =

1 AB 2 4

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Plane

1 2 c 4 1 b 2 + g 2 = a2 4

a2 +b2 =



g 2 +a 2 =



12-25

(12.52) (12.53)

1 2 b 4

(12.54)

(12.52) + (12.53) - (12.54) gives, 1 2a 2 = (b 2 + c 2 − a 2 ) 4 1 a 2 = (b2 + c 2 − a2 ) 8 1 a = b2 + c 2 − a2 2 2



b =

Similarly,

g =



1 2 2 1 2 2

(12.55)

c 2 + a2 − b2 

(12.56)

a2 + b2 − c 2

(12.57)

x y z Therefore, the equation of the plane is + + = 1 where a, b, g   are a b g given by (12.55), (12.56) and (12.57). Let ( x1 , y1 , z1 ),( x2 , y2 , z2 ) and ( x3 , y3 , z3 ) be the vertices of the ∆ ABC. Then x2 + x3 = 2a x3 + x1 = 0



x1 + x2 = 0 Adding we get 2( x1 + x2 + x3 ) = 2a or x1 + x2 + x3 = a ∴  x1 + 2a = a ∴ x1 = −a



∴ x2 = a Similarly,

and

x3 = a

y1 = b , y2 = − b , y3 = b

z1 = g , z2 = −g , z3 = g Therefore, the vertices of the triangle are (-a, b, g  ) (a, -b, g  ) and (a, b, -g  ).

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12-26

Analytical Geometry

Example 12.22 Find the angle between the planes 2x - y + z = 6, x + y + 2z = 3.

Solution The direction ratios of the normal to the planes are 2, -1, 1 and 1, 1, 2. The di2 −1 1 1 1 2 . If q is the angle rection cosines of the normal are , , , , , 6 6 6 6 6 6 between the planes, then cos q = ll1 + mm1 + nn1 =

2 6

.

1 6

−

1 6

.

1 6

+

1 6

.

2 6

=

3 1 = 6 2

p ∴q = 3

Example 12.23 Prove that the plane x + 2y + 2z = 0, 2x + y - 2z = 0 are at right angles.

Solution The direction ratios of the normals to the planes are 1, 2, 2 and 2, 1, -2. If the lines are to be perpendicular then a1a2 + b1b2 + c1c2 = 0. Hence, a1a2 + b1b2 + c1c2 = 2 + 2 − 4 = 0. Therefore, the normals are perpendicular and hence the planes are perpendicular.

Example 12.24 Find the equation of the plane containing the line of intersection of the planes x + y + z - 6 = 0, 2x + 3y + 4z + 5 = 0 and passing through the point (1, 1, 1).

Solution The equation of any plane containing the line is x + y + z - 6 = l (2x + 3y + 4z + 5) = 0. If this line passes through the point (1, 1, 1) then, 1 + 1 + 1 - 6 + l(2 + 3 + 4 + 5) = 0. 3 14l = 3 or l = . 14 3 Therefore, the equation of the required plane is x + y + z - 6 + (2x + 14 3y + 4z + 5) = 0.

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Plane



⇒14x + 14y + 14z - 84 + (6x + 9y + 12z + 15) = 0



20x + 23y + 26z - 69 = 0

12-27

Example 12.25 Find the equation of the plane which passes through the intersection of the planes 2x + 3y + 10z - 8 = 0, 2x - 3y + 7z - 2 = 0 and is perpendicular to the plane 3x - 2y + 4z - 5 = 0.

Solution The equation of any plane passing through the intersection of the planes 2x + 3y + 10z - 8 = 0 and 2x - 3y + 7z - 2 = 0 is 2x + 3y + 10z - 8 + l(2x -  3y + 7z - 2) = 0. The direction ratios of the normal to this plane are 2 + 2l, 3 - 3l, 10 + 7l. The direction ratios of the plane 3x - 2y + 4z - 5 = 0 are 3, –2, 4. Since these two planes are perpendicular, 3(2 + 2l) - 2 (3 -3l) + 4(10 + 7l) = 0. 6 + 6l - 6 + 6l + 40 + 28l = 0 or 40l = - 40 or l = -1



Therefore, the required plane is 2x + 3y + 10z - 8 - (2x - 3y + 7z - 2) = 0.

\ 6y + 3z - 6 = 0 or 2y + z - 2 = 0

Example 12.26 The plane x - 2y + 3z = 0 is rotated through a right angle about its line of intersection with the plane 2x + 3y - 4z + 2 = 0. Find the equation of the plane in its new position.

Solution The plane x - 2y + 3z = 0 is rotated about the line of intersection of the planes

x - 2y + 3z = 0

(12.58)



2x + 3y - 4z + 2 = 0

(12.59)

The new position of the plane (12.58) passes through the line of intersection of the two given planes. Therefore, its equation is

x - 2y + 3z + l(2x + 3y - 4z + 2) = 0

(12.60)

The plane (12.60) is perpendicular to the plane (12.58).

7 1(1 + 2l) - 2(–2 + 3l) + 3(3 - 4l) = 0 or –16l + 14 = 0 or l = . 8

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12-28

Analytical Geometry

Therefore, the equation of the plane (12.58) in its new position is x - 2y + 7 3z + (2x + 3y - 4z + 2) = 0. 8 (i.e.)  22x + 5y - 42 + 14 = 0

Example 12.27 The line lx + my = 0 is rotated about its line of intersection with the plane z = 0 through an angle a. Prove that the equation of the plane is lx + my ± z l 2 + m2 tana = 0.

Solution Any plane passing through the intersection of lx + my = 0 and z = 0 is lx + my + lz = 0



(12.61)

The plane lx + my + lz = 0 is rotated through an angle a along the plane (12.61).

∴ cos a =



cosa =

l.l + m.m + l .0 l + m2 l 2 + m2 + l 2 2

l 2 + m2 l 2 + m2 + l 2

l 2 + m2 + l2 = (l 2 + m2)sec2a ⇒ l2 = (l 2 + m2)(sec2a - 1)



= (l 2 + m2)tan2a



l = ± l 2 + m2 tan a Therefore, the equation of the plane in its new position is given by



lx + my ± z l 2 + m2 tana = 0

Example 12.28 Find the equation of the plane passing through the line of intersection of the planes 2x - y + 5z - 3 = 0 and 4x + 2y - z + 7 = 0 and parallel to z-axis.

Solution The equation of the plane passing through the line of intersection of the given planes is 2x - y + 5z - 3 + l(4x + 2y - z + 7) = 0. If the plane is parallel to

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Plane

12-29

z-axis, its normal is perpendicular to z-axis. The directions of the normal to the plane are 2 + 4l, - 1 + 2l, 5 - l. The direction ratios of the z-axis are 0, 0, 1.

∴  (2 + 4l)0 + (-1 + 2l)0 + (5 - l)1 = 0. ∴  l = 5

Hence, the equation of the required plane is (2x - y + 5z - 3) + 5(4x + 2y -  z + 7) = 0. 22x + 9y + 32 = 0

(i.e.)

Example 12.29 Find the distance of the points (2, 3, -5), (3, 4, 7) from the plane x + 2y -  2z = 9 and prove that these points lie on the opposite sides of the plane.

Solution Let the line joining the points P(2, 3, -5) and Q(3, 4, 7) be divided by the plane in the ratio l:1. Then

l=−

Ax1 + By1 + Cz1 + D 2 + 6 + 10 − 9 −9 3 =− = = 3 + 8 − 14 − 9 −12 4 Ax2 + By2 + Cz2 + D

= a positive quantity

Therefore, the points P and Q lie on the opposite side of the plane. The perpendicular distance from (2, 3, -5) to the plane x + 2y - 2z - 9 = 0 is p1 = 2 + 6 + 10 − 9 = 9 = 3 units. 3 1+ 4 + 4 The perpendicular distance from (3, 4, 7) to the plane is 3 + 8 − 14 − 9 −12 p2 = = = −4 units. 3 1+ 4 + 4

∴ 

p2 = 4 units

Note 12.29.1:  Since p1 and p2 are of opposite signs the points are on the opposite sides of the plane. Example 12.30 Prove that the points (2, 3, -5) and (3, 4, 7) lie on the opposite sides of the plane which meets the axes in A, B and C such that the centroid of the triangle A, B and C is the points (1, 2, 4).

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12-30

Analytical Geometry

Solution

x y z + + = 1. Then the coordinates of A, B a b c  a b c and C are (a, 0, 0), (0, b, 0) and (0, 0, c). The centroid is  , ,  . But the  3 3 3 centroid is given as (1, 2, 4). Let the equation of the plane be

a = 1 or a = 1 3 b = 2 or b = 6 3 c = 4 or c = 12. 3

∴ 



x y z + + = 1. 3 6 12 (i.e.)  4x + 2y + z = 12

Therefore, the equation of the plane ABC is

Let the line joining the points P(2, 3, -5) and Q(3, 4, 7) be divided by the plane in the ratio l:1. 8 + 6 − 5 − 12 −3 1 Then l = − = =− . 12 + 8 + 7 − 12 15 5 Therefore, the points lie on the opposite sides of the plane.

Example 12.31 Find the distance between the parallel planes 2x - 2y + z + 3 = 0, 4x - 4y + 2z + 5 = 0.

Solution Let (x1, y1, z1) be a point on the plane 2x - 2y + z + 3 = 0 ∴  2x1 - 2y1 + z1 + 3 = 0



(12.62)

Then the distance between the parallel planes is equal to the distance from (x1, y1, z1) to the other plane.

=±

4 x1 − 4 y1 + 2 z1 + 5 4 +4 +2 2

2

2

=±

( −6 + 5) 1 =   using (12.62) 6 6

Note 12.31.1:  The distance between the parallel planes ax + by + cz + d = 0 and ax + by + cz + d1 = 0 is

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d − d1

a + b2 + c 2 2

.

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Plane

12-31

On dividing the equation 4x - 4y + 2z + 5 = 0 by 2, we get 2x − 2y + z +



5 =0 2

Distance between the planes =

5  1   2 =  2 = 1. 3 6 9

3−

Example 12.32

 1 A plane is drawn through the line of x + y = 1, z = 0 to make an angle sin-1  3  with the plane x + y + z = 0. Prove that two such planes can be drawn. Find  7 their equation. Show that the angle between the planes is cos-1  9  .

Solution The equation of the plane through the line x + y = 1, z = 0 is x + y - 1 + lz = 0



(12.63)

The direction ratios of this plane is 1, 1, l. Also the direction ratios of the plane x + y + z = 0 are 1, 1, 1. If q is the angle between these two planes then

cos q =



cos 2 q =



1−

1+1+ l 1 +1 + l 2

2

( l + 2)2 3l 2 + 6

2

1 +1 +1 2

2

2

⇒ 1 − sin 2 q =

=

l +2 3( l 2 + 2)

.

l 2 + 4l + 4 3l 2 + 6

1 l 2 + 4l + 4 8 l 2 + 4l + 4 = ⇒ = 2 9 9 3l + 6 3l 2 + 6

2 or 5l2 - 12l + 4 = 0 → (l - 2)(5l - 2) = 0 → l = 2 or . 5 From (12.63), the equations of the required planes are x + y + 2z = 1 and 5x + 5y + 2z - 5 = 0. If q is the angle between these two planes then 5 + 5 + 4 14 cosq = = . 6 54 18  7 ∴q = cos −1    9



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12-32

Analytical Geometry

Example 12.33 Find the bisectors of the angles between the planes 2x - y + 2z + 3 = 0, 3x -  2y + 6z + 8 = 0; also find out which plane bisects the acute angle.

Solution The two given planes are 2x - y + 2z + 3 = 0 3x - 2y + 6z + 8 = 0



The equations of the bisectors are

(12.64) (12.65)

2x − y + 2z + 3 4 +1+ 4

=±

3x − 2 y + 6 z + 8 9 + 4 + 36

7(2 x − y + 2 z + 3) = ±3(3 x − 2 y + 6 z + 8) (14 x − 7 y + 14 z + 21) = ±(9 x − 6 y + 18 z + 24)



(i.e.)  5x - y - 4z - 3 = 0  23x - 13y + 32z + 45 = 0

(12.66) (12.67)

Let q be the angle between the planes (12.64) and (12.66) then 10 + 1 − 8



cosq =



tan q  = sec q - 1 = 42 - 1 = 41



2

25 + 1 + 16 4 + 1 + 4

=

1 42

2

∴ tanq = 41 > 1

Hence q  > 45°. The plane 5x - y - 4z - 3 = 0 bisects the obtuse angle between the planes (12.64) and (12.65). Therefore, 23x - 13y + 32z + 45 = 0 bisects the acute angle between the planes (12.64) and (12.65).

Example 12.34 Prove that the equation 2x2 - 2y2 + 4z2 + 2yz + 6zx + 3xy = 0 represents a pair  4

of planes and angle between them is cos-1  9  .

Solution

3 Here a = 2, b = –2, c = 4,  f = 1, g = 3, h = . 2 Now, abc + 2fgh - af 2 - bg2 - ch2 = 0

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Plane

12-33

⇒ –16 + 9 - 2 + 18 - 9 = 0



Hence, the given equation represents a pair of planes. Let q be the angle between the planes. Then tanq = ±

2 f 2 + g 2 + h2 − bc − ca − ab a+b+c

2 1+ 9 + 9 + 8 − 8 + 4 65 4 tanq = ± = 4 4



sec2q = 1 + tan2q =

81 4 and cosq = 16 9  4 ∴q = cos −1    9



Exercises Section A 1.  2.  3.  4.

5.  6. 

If P is the point (2, 3, -1), find the equation of the plane passing through P and perpendicular to OP. Ans.: 2x + 3y - z - 14 = 0 The foot of the perpendicular from the origin to a plane is (12, -4, -3). Find its equation. Ans.: 12x - 4y -3z + 69 = 0 Find the intercepts made by the plane 4x - 3y + 2z - 7 = 0 on the coordinate axes. 7 −7 7 Ans.: , , 4 3 2 A plane meets the coordinate axes at A, B and C such that the centroid of the triangle is the point (a, b, c). Show that the equation of the plane is x y z + + = 3. a b c Find the equation of the plane that passes through the point (2, -3, 1) and is perpendicular to the line joining the points (3, 4, -1) and (2, -1, 5). Ans.: x + 5y - 6z + 19 = 0 O is the orgin and A is the point (a, b, c). Find the equation of the plane perpendicular to A. Ans.: ax + by + cz - (a2 + b2 + c2) = 0

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12-34

Analytical Geometry

7. 

Find the equation of the plane passing through the points: (i) (8, -2, 2), (2, 1, -4), (2, 4, -6) (ii) (2, 2, 1), (2, 3, 2), (-1, 3, 0) (iii) (2, 3, 4), (-3, 5, 1), (4, -1, 2) Ans.: (i) 2x - 2y - 2z = 14, (ii) 2x + 3y - 3z - 7 = 0, (iii) x + y –z - 1 = 0

8.

Show that the points (0, -1, -1), (4, 5, 1),(3, 9, 4) and (-4, 4, 4) lie on a plane.

9.

Show that the points (0, -1, 0), (2, 1, -1), (1, 1, 1) and (-3, 3, 0) are coplanar.

10.

Find the equation of the plane through the three points (2, 3, 4), (-3, 5, 1) and (4, -1, 2). Also find the angles which the normal to the plane makes with the axes of reference.  1   1   −1  Ans.: x + y - z - 1 = 0;cos −1   ,cos −1   ,cos −1    3  3  3

 11.  12.  13.  14.  15.  16.  17.  18.  19.

Find the equation of the plane which passes through the point (2, -3, 4) and is parallel to the plane 2x - 5y - 7z + 15 = 0. Ans.: 2x - 5y - 7z + 9 = 0 Find the equation of the plane through (1, 3, 2) and perpendicular to the planes x + 2y + 2z - 5 = 0 and 3x + 3y + 3z - 8 = 0. Ans.: 2x - 4y + 3z + 8 = 0 Find the equation of the plane which passes through the point (2, 2, 4) and perpendicular to the planes 2x - 2y - 4z + 3 = 0 and 3x + y + 6z - 4 = 0. Ans.: x - 3y - z - 4 = 0 Find the equation of the plane which passes through the points (9, 3, 6) and (2, 2, 1) and perpendicular to the plane 2x + 4y + 6z - 9 = 0. Ans.: 3x + 4y - 5z - 9 = 0 Find the equation of the straight line passing through the points (-1, 1, 1) and (1, -1, 1) and perpendicular to the plane x + 2y + 2z - 5 = 0. Ans.: 2x + 2y - 3z + 3 = 0 Find the equation of the plane which passes through the points (2, 3, 1), (4, -5, 3) and are parallel to the coordinate axes. Ans.: y + 4z - 7 = 0, x - z - 1 = 0, 4x + y - 11 = 0 Find the equation of the plane which passes the point (1, 2, 3) and parallel to 3x + 4y - 5z = 0. Ans.: 3x + 4y - 5z + 4 = 0 Find the equation of the plane bisecting the line joining the points (2, 3, -1) and (-5, 6, 3) at right angles. Ans.: x - y - z + 7 = 0 A variable plane is at a constant distance p from the origin and meets the axes in A, B and C. Show that the locus of centroid of the tetrahedron OABC is x-2 + y-2 + z-2 = 16p-2.

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Plane

12-35

20.

OABC is a tetrahedron in which OA, OB and OC are mutually perpendicular. Prove that the perpendicular from O to the base ABC meets it at its orthocentre.

21.

Through the point P(a, b, c) a plane is drawn at right angles to OP to meet the p5 where p is axes in A, B and C. Prove that the area of the triangle ABC is 2abc the length of OP.

22.

A plane contains the points A(-4, 9, -9) and B(5, -9, 6) and is perpendicular to the line which joins B and C(4, -6, k). Obtain k and the equation of the plane. 51 Ans.: k = , 5x - 15y - 21z = 34 5

 23.  24.  25.  26.

Find the distance between the parallel planes 2x + y + 2z - 8 = 0 and 4x + 2y + 4z + 5 = 0. 7 Ans.: 2 Find the locus of the point, the sum of the squares of whose distances from the planes x + y + z = 0, x = z = 0, x - 2y + z = 0 is 9. Ans.: x2 + y2 + z2 = 9 Find the equation of the plane which is at a distance 1 unit from the origin and parallel to the plane 3x + 2y - z + 2 = 0. Ans.: 3x + 2y z ± 14 = 0 x y z The plane + + = 1 meets the coordinate axes in A, B and C, respectively. a b c 1 1 Show that the area of the triangle ABC is [b 2c 2 + c 2 a 2 + a 2b 2 ]2 . 2

27.

Show that the equations by + cz + d = 0, cz + ax + d = 0, ax + by + d = 0 represent planes parallel to OX, OY and OZ, respectively.

28.

Show that the points (2, 3, -5) and (3, 4, 7) lie on the opposite sides of the plane meeting the axes in A, B and C such that the centroid of the triangle ABC is the point (1, 2, 4).

29.

Find the locus of the point such that the sum of the squares of its distances from the planes x + y + z = 0 and x - 2y + z = 0 is equal to its distance from the plane x - z = 0. Ans.: y2 - 2xz = 0

 30.  31.

Find the locus of the point whose distance from the origin is 7 times its distance from the plane 2x + 3y - 6z = 2. Ans.: 3x2 + 8y2 + 53z2 - 36yz - 24zx + 12xy - 8x - 12y + 24z + 14 = 0 Prove that the equation of the plane passing through the points (1, 1, 1), (1, -1, 1) and (-7, -3, -5) and is parallel to axis of y.

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12-36

32.



Analytical Geometry

Determine the constant k so that the planes x - 2y + kz = 0 and 2x + 5y - z = 0 are at right angles, and in that case find the plane through the point (1, -1, -1) and perpendicular to both the given planes. Ans.: k = -8, 14x - 5y + 3z - 16 = 0

33.

Prove that 3x - y - z + 11 = 0 is the equation of the plane through (-1, 6, 2) and perpendicular to the join of the points (1, 2, 3) and (-2, 3, 4).

34.

A, B and C are points (a, 0, 0), (0, b, 0) and (0, 0, c). Find the equation of the plane through BC which bisects OA. By symmetry write down the equations of the plane through CA bisecting OB and through AB bisecting OC. Show  a b c that these planes pass through  , ,  .  4 4 4

Section B 1.

 2.  3.

 4.  5.  6.

Find the equation of the plane through the intersection of the planes x + 3y + 6 = 0 and 3x - y - 4z = 0 whose perpendicular distance from the origin is unity. Ans.: 2x + y - 2z + 3 = 0, x - 2y - 2z - 3 = 0 Find the equation of the plane through the intersection of the planes x - 2y + 3z + 4 = 0 and 2x - 3y + 4z - 7 = 0 and the point (1, -1, 1). Ans.: 9x - 13y - 17z - 39 = 0 Find the equation of the plane through the intersection of the planes x + 2y + 3z + 4 = 0 and 4x + 3y + 3z + 1 = 0 and perpendicular to the plane x + y + z + 9 = 0 and show that it is perpendicular to xz-plane. Ans.: x - z = 2 Find the equation of the plane through the point (1, -2, 3) and the intersection of the planes 2x - y + 4z - 7 = 0 and x + 2y - 3z + 8 = 0. Ans.: 17x + 14y + 11z + 44 = 0 Find the equation of the plane passing through the intersection of the planes x + 2y + 3z + 4 = 0 and 4x + 3y + 2z + 1 = 0 and through the point (1, 2, 3). Ans.: 11x + 4y - 3z - 10 = 0 Find the equation of the plane passing through the line of intersection of the planes x - 2y - z + 3 = 0 and 3x + 5y - 2z - 1 = 0 which is perpendicular to the yz-plane. Ans.: 11y + z - 10 = 0

 7.

The plane x + 4y - 5z + 2 = 0 is rotated through a right angle about its line of intersection with the plane 3x + 2y + z + 1 = 0. Find the equation of the plane in its new position.



M12_ANALYTICALGEOMETRY_ISBN_CH12.indd 36

Ans.: 20x + 10y + 12z + 5 = 0

4/22/2013 8:15:40 PM

Plane

8.

Are the planes given by the equations 3x + 4y + 5z + 10 = 0 and 9x + 12y + 15z + 20 = 0 parallel? If so find the distance between them. Ans.:

 9.

 10.

 11.

 12.

 13.

 14.  15.  16.

 17. 

12-37

2 3

Find the equation of the plane passing through the line of intersection of the planes 2x - y = 0 and 3x - y = 0 and are perpendicular to the plane 4x + 3y - 3z = 8. Ans.: 24x - 17y + 15z = 0 Find the equation of the plane passing through the line of intersection of the planes ax + by + cz + d = 0 and a1 x + b1 y + c1 z + d1 = 0 perpendicular to xy-plane. Ans.: ( ac1 − a1c) x + (bc1 − b1c) y + ( dc1 − d1c) z = 0 Find the equation of the plane passing through the line of intersection of the planes 2x + 3y + 10z - 8 = 0, 2x - 3y + 7z - 2 = 0 and is perpendicular to the plane 3x - 2y + 4z - 5 = 0. Ans.: 2y + z - 2 = 0 Obtain the equation of the planes bisecting the angles between the planes x + 2y - 2z + 1 = 0 and 12x - 4y + 3z + 5 = 0. Also show that these two planes are at right angles. Ans.: 23x - 38y + 35z + 2 = 0  49x + 14y - 17z + 28 = 0 Find the equation of the plane that bisects the angle between the planes 3x - 6y - 2z + 5 = 0 and 4x - 12y + 3z - 3 = 0 which contain the origin. Does this plane bisect the acute angle? Ans.: yes, 67x + 162y + 47z + 44 = 0 Find the equation of the plane that bisects the acute angle between the planes 3x - 4y + 12z - 26 = 0 and x + 2y - 2z - 9 = 0. Ans.: 22x + 14y + 10z - 195 = 0 Find the equation of the plane that bisects the obtuse angle between the planes 4x + 3y - 5z + 1 = 0 and 12x + 5y - 13 = 0. Ans.: 8x - 14y - 13 = 0 Show that the origin lies in the acute angle between the planes x + 2y - 2z -   9 = 0, 4x - 3y + 12z + 13 = 0. Find the planes bisecting the angle between them and find the plane which bisects the acute angle. Ans.: x + 35y - 10z - 156 = 0 Find the equation of the plane which bisects the acute angle between the planes x + 2y + 2z - 3 = 0 and 3x + 4y + 12z + 1 = 0. Ans.: 11x + 19y + 31z - 18 = 0

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12-38

18.

Analytical Geometry

Prove that the equation 2 x 2 − 6 y 2 − 12 z 2 + 18 yz + 2 zx + xy = 0 represents a  16  pair of planes and show that the angle between them is cos −1   .  21 3 4 5 + + = 0 represents a pair of planes. y−z z−x x−y

19.

Prove that the equation

20.

If the equation f (x, y, z) = ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0 represents a pair of planes then prove that the product of the distances of the planes from f (a , b ,g ) (a, b, g  ) is . 2 a + ∑ 4∑ f 2 − 2∑ bc

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Chapter

13

Straight Line

13.1  Introduction The intersection of two planes P1 and P2 is the locus of all the common points on both the planes P1 and P2. This locus is a straight line. Any given line can be uniquely determined by any of the two planes containing the line. Thus, a line can be regarded as the locus of the common points of two intersecting planes. Let us consider the two planes ax + by + cz + d = 0 a1x + b1y + c1z + d1 = 0



(13.1) (13.2)

Any set of coordinates (x, y, z) which satisfy these two equations simultaneously will represent a point on the line of intersection of these two planes. Hence these two equations taken together will represent a straight line. It can be noted that the equation of x-axis are y = 0, z = 0. The equation of the y-axis is x = 0, z = 0 and the equation of the z-axis is x = 0, y = 0. The representation of the straight line by the equations ax + by + cz + d = 0 and a1x + b1y + c1z + d1 = 0 is called non-symmetrical form. Let us now derive the equations of a straight line in the symmetrical form.

13.2  Equation of a straight line in symmetrical form Let A(x1, y1, z1) be a point on the straight line and P(x, y, z) be any point on the straight line. Let l, m, n be the direction cosines of the straight line. Let OP = r. The projections of AP on the coordinate axes are x - x1, y - y1, z - z1. Also the projections of AP on the coordinate axes are given by lr, mr, nr. Then x - x1 = lr, y - y1 = mr and z - z1 = nr.

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13-2

Analytical Geometry

∴



x − x1 y − y1 z − z1 = = =r l m n z

z ) , y 1, 1 A(x 1

P(x, y, z)

y

x

These equations are called the symmetrical form of the straight line.

Aliter: We now derive the equations in symmetrical form from the vector equation of the straight line passing through a point and parallel to a vector. A

P →

r O

Let A be a given  point on a straight line and P be any point on the straight line. Let b be a vector parallel to the line. Let O be the origin and     OA = a , OP = r . Then      AP = OP − OA = r − a (13.3) But where t is a scalar. From (13.3) and (13.4),

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 2

  AP = tb

(13.4)

      r − a = tb or r = a + tb

(13.5)

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Straight Line

13-3

This equation is true for all positions of P on the straight line and line. Let  therefore   this  is the  vector  equation  of the  straight  OP = xi + y j + zk , OA = x1 i + y1 j + z1 k and b = li + m j + nk . Then from equation (13.4), we have        ( x − x1 )i + ( y − y1 ) j + ( z − z1 )k = tb = t (li + mj + nk )    Equating the coefficients of i , j and k , we have

x - x1 = tl,  y - y1 = tm,  z - z1 = tn ∴

x − x1 y − y1 z − z1 = = =t l m n

These are the cartesian equations of the straight line in symmetrical form.

Note 13.2.1: To express the equations of a straight line in symmetrical form we require (i) the coordinate of a point on the line and (ii) the direction ­cosines of the straight line.

Note 13.2.2: Any point on this line is (x1 + lr, y1 + mr, z1 + nr). Even if l, m

and n are the direction ratios of the line, (x1 + lr, y1 + mr, z1 + nr) will represent a point on the line but r will not be distance between the points (x, y, z) and (x1, y1, z1).

13.3  Equations of a straight line passing through the two given points Let P(x1, y1, z1) and Q(x2, y2, z2) be two given points. The direction ratios of the line are x2 - x1, y2 - y1, z2 - z1. x − x1 y − y1 z − z1 Therefore, the equations of the straight line are = = . x2 − x1 y2 − y1 z2 − z1

Aliter:

Let O be the origin and P and Q be the points on the straight line and R be any point on the straight line. P →

Q →

a

b

R →

r

O

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13-4

Analytical Geometry

      OP = a , OQ = b , OR = r           Then PQ  =OQ  − OP = b − a. Also PR = OR − OP = r − a . But PR = t PQ   (i.e.)  r − a = t (b − a )     r = a + t (b − a )

(13.6)

This is the vector equation of the straight line.             Let r = xi + yj + zk , a = x1i + y1i + z1k , b = x2 i + y2 j + z2 k . Then from (13.6), we get     ( x − x1 )i + ( y − y1 ) j + ( z − z1 )k = t [( x2 − x1 )i +   ( y2 − y1 ) jthe + (coefficients z2 − z1 )k ] of i, j and k, we get Equating x - x1 = t(x2 - x1); y - y1 = t(y2 - y1); z - z1 = k(z2 - z1)



∴

x − x1 y − y1 z − z1 = = =t x2 − x1 y2 − y1 z2 − z1

13.4  Equations of a straight line determined by a pair of planes in symmetrical form We have already seen that a straight line is determined by a pair of planes ax + by + cz + d = 0 and a1x + b1y + c1z + d1 = 0 we now express these equations in symmetrical form. To find it we need to find the direction cosines of the line and the coordinates of a point on the line. Let l, m, n be the direction cosines of the line. This line is perpendicular to the normal to the two given planes since the line lies on the plane. The direction ratios of the two normals are a1, b1, c1 and a2, b2, c2. The direction cosines of the line are l, m, n. Since the normals are perpendicular to the line we have,

al + bm + cn = 0

(13.7)



a1l + b1m + c1n = 0

(13.8)

l m n = = . bc1 − b1c ca1 − a1c ab1 − b1a Therefore, the direction ratios of the line are

Solving for l, m, n we get



bc1 - b1c, ca1 - a1c, ab1 - a1b

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 4

(13.9)

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Straight Line

13-5

To find a point on the line, let us find the point where the line meets the plane. z = 0 and a1x + b1y + d = 0 and a1x + b1y + d1 = 0. Solving the last two equations, we get x y 1 = = bd1 − b1d a1d − ad1 ab1 − a1b



 bd − b d a d − ad1  Therefore, a point on the line is  1 1 , 1 ,0 .  ab1 , a1b ab1 − a1b  Then the equations of the bd − b d a d − ad1 x− 1 1 y− 1 ab1 − ba1 ab1 − a1b z−0 = = . bc1 − b1c ca1 − a1c ab1 − a1b

straight

lines

are

Note 13.4.1: We can also find the point where the line meets the yz-plane or zx-plane.

13.5  Angle between a plane and a line Let the equation of the plane be ax + by + cz + d = 0. x − x1 y − y1 z − z1 = = Let the equation of the line be . l m n Let q  be the angle between the plane and the line. The direction ratios of the normal to the plane are a, b, c. The direction ratios of the line are l, m, p  n. Since q  is the angle between the plane and the line,  − q  is the angle 2  between the normal to the plane and the line.

sinq =

p  ∴ cos  − q  = 2 

al + bm + cn a + b 2 + c 2 l 2 + m 2 + n2 2

  al + bm + cn or q = sin −1    a 2 + b 2 + c 2 q 2 + m2 + n2  l 2 + m 2 + n2

al + bm + cn a2 + b2 + c2

Note 13.5.1: If the line is parallel to the plane, q  = 0.

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∴ al + bm + cn = 0

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13-6

Analytical Geometry

13.6  Condition for a line to be parallel to   a plane Let the equation of the plane be ax + by + cz + d = 0



(13.10)

Let the equation of the line be

x − x1 y − y1 z − z1 = = l m n

(13.11)

If the line is parallel to the plane then the normal to the plane is perpendicular to the line. The condition for this is al + bm + cn = 0



(13.12)

Since (x1, y1, z1) is a point on the line and does not lie on the plane given by (13.10). \ ax1 + by1 + cz1 + d ≠ 0 Hence the conditions for the line (13.11) to be parallel to the plane (13.10) are al + bm + cn = 0 and ax1 + by1 + cz1 + d ≠ 0.

13.7  Conditions for a line to lie on a plane Let the equation of the line be

x − x1 y − y1 z − z1 = = l m n

(13.13)

Let the equation of the plane be ax + by + cz + d = 0



(13.14)

Since the line lies on the plane,

al + bm + cn = 0

(13.15)

Since the line lies on the plane every point on the line is also a point on the plane. (x1, y1, z1) is a point on the line and therefore it should also lie on the plane given by (13.14). Hence, ax1 + by1 + cz1 + d = 0. Therefore, the conditions for the line (13.13) to be parallel to the plane (13.14) are al + bm + cn = 0 and ax1 + by1 + cz1 + d = 0.

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13-7

Straight Line

13.8  To find the length of the perpendicular from a given point on a line Let the given point be P(  p, q, r) and the given line QR be x − x1 y − y1 z − z1 = = l m n



(13.16)

P(p, q, r)

Q

L (x1, y1, z1)

M

R

Then L(x1, y1, z1) is a point on the line. Draw PM perpendicular to the line.

PM 2 = PL2 - LM 2

(13.17)

PL = (x1 - p) + (y1 - q) + (z1 - r) 2

2

2

2

Also LM is the projection of PL on QR. \ LM is (x1 - p)l + (y1 - q)m + (z1 - r)n

(13.18)

Then from (13.17), PM 2 = (x1 - p)2 + (y1 - q)2 + (z1 - r)2 - [l(x1 - p) + m(y1 - q) + n(z1 - r)]2      PM 2 = [(y1 - q)n + (z1 - r)m]2 + [(z1 - r)l - (x1 - r)n]2         - [(x1 - p)m (y1 - q)l]2 (13.19) (using Lagrange’s identity)

13.9  Coplanar lines

x − x1 y − y1 z − z1 = = = r1 and Find the condition for the lines l1 m1 n1 x − x2 y − y2 z − z2 = = = r2 to be coplanar and also find the equation of ls m2 n2 the plane containing these two lines. Consider equations,

x − x1 y − y1 z − z1 = = = r1 l1 m1 n1

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(13.20)

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13-8

Analytical Geometry

x − x2 y − y2 z − z2 = = = r2 l2 m2 n2



(13.21)

Let the equation of the plane be ax + by + cz + d = 0



(13.22)

Since the planes contains lines (13.20), we have

ax1 + by1 + cz1 + d = 0

(13.23)



al1 + bm1 + cn1 = 0

(13.24)

From (13.20) and (13.21), we get a(x - x1) + b(y - y1) + c(z - z1) = 0



(13.25)

Since the plane also contains the line (13.21) the point (x2, y2, z2) lies on the plane (13.22). \ a(x2 - x1) + b(y2 - y1) + c(z2 - z1) = 0 Also al2 + bm2 + cn2 = 0



(13.26) (13.27)

Eliminating a, b, c from equation (13.24), (13.26) and (13.27), we get

x2 − x1 l1 l2

y2 − y1 m1 m2

z2 − z1 n1 = 0 n2

This is the required condition for the lines (13.20) and (13.21) to be coplanar. Eliminating a, b, c from the equation (13.23), (13.24) and (13.25), we get the equation of the plane containing the two given lines as x − x1 y − y1 z − z1 l1 m1 n1 = 0. l2 m2 n2

Aliter: If the planes are coplanar they may intersect. Any point on the line (13.20) is x1 + l1r1, y1 + m1r1, z1 + n1 r1. Any point on the line (13.21) is x2 + l2r2, y2 + m2r2, z2 + n2 r2. If the two lines intersect then the two points are the same. \ x1 + l1r1 = x2 + l2r2 y1 + m1r1 = y2 + m2r2

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Straight Line

13-9

z1 + n1r1 = z2 + n2r2



\ (x1– x2) + (lr1 - l2r2) = 0

(y1 - y2) + (m1r1 - m2r2) = 0



(z1 - z2) + (n1r1 - n2r2) = 0 Eliminating r1 and r2 from the above equations, we get



x1 − x2 y1 − y2 z1 − z2

l1 m1 n1

l2 m2 = 0 (i.e.) n2

x2 − x1 l1 l2

y2 − y1 m1 m2

z2 − z1 n1 = 0 n2

This is the required condition for coplanar lines.

13.10  Skew lines Two non-intersecting and non-parallel lines are called skew lines. There also exists a shortest distance between the skew lines and the line of the shortest distance which is common perpendicular to both of these.

13.10.1  Length and Equations of the Line of the Shortest Distance Let the equation of the skew lines be

x − x1 y − y1 z − z1 = = l1 m1 n1

(13.28)



x − x2 y − y2 z − z2 = = l2 m2 n2

(13.29)

Let PQ be the line of the shortest distance between lines (13.28) and (13.29). Let l, m, n be the direction cosines of the lines of the shortest distance PQ. A P

l (x1, y1, z1) B

D Q

m (x2, y2, z2)

C

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13-10

Analytical Geometry

The condition for PQ to be perpendicular to AB and CD are ll1 + mm1 + nn1 = 0 ll2 + mm2 + nn2 = 0 l m n = = . m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1 Therefore, the direction ratios of the line PQ are m1n2 - m2n1, n1l2 - n2l1, l1m2 - l2m1. Therefore, the direction ratios of the line of the SD are m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1 , , . 2 ∑ (m1n2 − m2 m1 ) ∑ (m1n2 − m2 m1 )2 ∑ (m1n2 − m2 m1 )2 Solving these two, we get

l(x1, y1, z1) and m(x2, y2, z2) are points on the lines (13.28) and (13.29). Then the length of the SD = PQ = Projection of LM on PQ = (x1 - x2)l + (y1 - y2)m + (z1 - z2)n, where l, m, n are the direction cosines of the line PQ.

=

( x1 − x2 )( m1n2 − m2 n1 ) + ( y1 − y2 )( n1l2 − n2 l1 ) + ( z1 − z2 )(l1m2 − l2 m1 )

∑ (m n

1 2



x1 − x2 = l1 l2

y1 − y2 m1 m1

z1 − z2 n1 ÷ n2

− m2 n1 )

∑ (m n

1 2

2

− m2 n1 )2

(13.30)

The equation of the plane containing the lines AB and PQ is x − x1 l l1



y − y1 m m1

z − z1 n = 0 n1

(13.31)

The equation of the plane containing the lines CD and PQ is x − x2 l l2



y − y2 m m2

z − z2 n = 0 n2

(13.32)

Therefore, the equation of the line of the SD is the intersection of these two planes and its equations are given by

x − x1 l l1

y − y1 m m1

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z − z1 x − x2 n =0= l n1 l2

y − y2 m m2

z − z2 n n2

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Straight Line

13-11

Note 13.10.1: If the lines (13.28) and (13.29) are coplanar then the SD ­between the lines is zero. Hence the condition for the lines (13.28) and x − x1 y − y1 z − z1 (13.29) to be coplanar, from (13.30) is l1 m1 n1 = 0. l2 m2 n2 Aliter: Let the vector equations of the two lines be    r = a1 + tb1    r = a2 + sb2 where t and s are scalars.     a1 = x1i + y1 j + z1k ,     b1 = l1i + m1 j + n1k ,

(13.33) (13.34)

    a2 = x2 i + y2 j + z2 k     b2 = l2 i + m2 j + n2 k

If the lines (13.33) and (13.34) are coplanar then the plane is parallel to the      vectors b1 and b2. Thereby b1 × b2 is perpendicular to the plane containing b1      and b2. Also as a1 and a2 are the points on the plane, a1 − a2 is a line on the plane       and is perpendicular to b1 × b2 . The condition for this is ( a1 − a2 ) ⋅ (b1 × b2 ) = 0     or [a1 − a2 , b1 , b2 ] = 0. x1 − x2 y1 − y2 z1 − z2 The scalar form of the equation is l1 m1 n1 = 0 . The l2 m2 n2     vector equation of the plane containing the two lines is ( r − a ) ⋅ (b1 × b2 ) = 0     or [r − a , b1 , b2 ] = 0.      But r − a = ( x − x1 )i + ( y − y1 ) j + ( z − z1 )k . x − x1 y − y1 z − z1 Therefore, the scalar equation of the plane is l1 m1 n1 = 0. l2 m2 n2

Aliter:

   Let the  vector equation of the two lines be r = a1 + tb1 and   r = a2 + sb2.             Let a1 = x1i + y1 j + z1k , a2 = x2 i + y2 j + z2 k and b1 = l1i + m1 j + n1k ,     b2 = l2 i + m2 j + n2 k .  Let DQ PQ is perpendicular  the lines AB  be the  SD between  at CD. Then   to both b1 and b2. Then PQ is parallel to b1 × b2 . Let a1 and a2 be the position vectors of points L and M on AB and CD, respectively.

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13-12









Analytical Geometry

SD = PQ = projection of LM on PQ     = the projection of (a1 − a2 ) on (b1 × b2 )     ( a1 − a2 ) ⋅ (b1 × b2 )    = b1 × b2     ( a1 − a2 )b1 , b2 )    = b1 × b2 x1 − x2 y1 − y2 z1 − z2   = l1 m1 n1 ÷ b1 × b2 l2 m2 n2    i j k      b1 × b2 = l1 m1 m1 = i ( m1n2 − m2 n1 ) + j (l1n2 − l2 n1 ) + k (ll1m2 − l2 m1 ) l2 m2 n2   b1 × b2 = Σ ( m1n2 − m2 n1 )2 x1 − x2 ∴ SD = l1 l2

y1 − y2 m1 m2

z1 − z2 n1 ÷ Σ( m1n2 − m2 n1 )2 n2

13.10.2  Equation of the Line of SD The equation of the line of the shortest distance is the equation of the line of intersection of the planes through the given    lines and the SD. The equation   of the plane containing the line r = a1 + tb1 and the SDPQ is parallel to b1 × b2    and therefore perpendicular to b1 × (b1 × b2 )is      ( r − a1 ) ⋅ b1 × (b1 × b2 ) = 0      r1 − a1 , b1 , b1 × b2  = 0 (13.35)    Similarly the equation of the plane containing the line r = a2 + sb2 and PQ is       r − a2 , b2 × (b1 × b2 ) = 0     r − a2 , b2 , b1 × b2  = 0 (13.36) The equation of the line of SD is the equation of the line of intersection of (13.35) and (13.36).       r − a1 , b2 , b1 × b2  = 0 (i.e.)

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Straight Line

13-13

In scalar forms, the equations of the line are x − x1 l1 m1n2 − m2 n1

y − y1 m1 n1l2 − n2 l1

z1 n1 =0 l1m2 − l2 m1

and x − x2

y − y2

l2 m1n2 − m2 n1

m2 n1l2 − n2 l1

z − z2 n2 = 0. l1m2 − l2 m1

13.11  Equations of two non-intersecting lines We will now show that the equations of any two skew lines can be part into the form y = mx, z = c and y = -mx, z = -c. A

L

B Q

C q q

y

x P

C

D M

C

Let AB and CD be two skew lines. Let LM be the common perpendicular to the skew lines. Let LM = 2c and q be its middle point. Choose O be the origin and draw lines OP and OQ parallel to AB and CD, respectively. Let the bisectors of POQ be chosen as axes of x and y. Let OE be taken as z-axis. Let POQ = 2q so that POX = XOQ = q . p p  Then the line OP makes angle q,  − q  and with x-, y-, z-axes. Its 2  2 direction cosines are cosa, sina, 0. The coordinates of L are (0, 0, c). AB is a straight line passing through L and parallel to AB. The equations of the line x−0 y−0 z−c OP are = = or y = xtanq, z = c cos a sin q 0 (i.e.)  y = mx, z = c where m = tanq

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13-14

Analytical Geometry

p p − q and with x-, y-, z-axes. 2 2 The direction cosines of the line OQ are cosq, -sinq, q. The line OQ makes angles -q,

The coordinates of M are (0, 0, -c). CD is a straight line passing through F x−0 y−0 z+c and parallel to CD. Its equations are = . = cos q − sin q 0 (i.e.)  y = -mx, z = -c where m = tanq

Note 13.11.1: Any point on the line AB is (r1, mr1, c) and on axis (r1, -mr1, -c).

13.12  Intersection of three planes Three planes may intersect in a line or a point. Let us find the conditions for three given planes to intersect (i) in a line and (ii) in a point. Let the equations of three given planes be

a1 x + b1 y + c1 z + d1 = 0

(13.37)

a2 x + b2 y + c2 z + d2 = 0

(13.38)

a3 x + b3 y + c3 z + d3 = 0

(13.39)

The equation of any plane passing through the intersection of planes (13.37) and (13.38) is (13.40) a1x + b1y + c1z + d1 + l (a2x + b2y + c2z + d2) = 0 If planes (13.37), (13.38) and (13.39) intersect in a line then equations (13.39) and (13.40) represent the same plane for same values of l. Identifying equations (13.40) and (13.39), we get

a1 + l a2 b1 + lb2 c1 + lc2 d1 + l d2 = = = = m( say ) a3 b3 c3 d3 a1 + la2 − ma3 = 0



b1 + lb2 − mb3 = 0 c1 + lc2 − mc3 = 0 d1 + ld2 − md3 = 0

Eliminating l and m from the equation taken three at a time, we get a1 a2 a3 a1 a2 a3 b1 b2 b3 = 0, b1 b2 b3 = 0 c1 c2 c3 d1 d2 d3

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Straight Line

a1   c1 d1

a2 c2 d2

a3 c3 = 0 and d3

b1 c1 d1

(i.e.)

a1 ∆1 = a2 a3

b1 b2 b3

c1 a1 c2 = 0, ∆ 3 = a2 c3 a3



a1 ∆ 2 = a2 a3

c1 c2 c3

d1 d2 = 0 and d3

b2 c2 d2 b1 b2 b3

b1 ∆ 4 = b2 b3

13-15

b3 c3 = 0 d3 d1 d2 = 0 d3 c1 d2 c3

d1 c2 = 0 d3

Therefore, the conditions for the three planes to intersect in a line are ∆1 = 0, ∆2 = 0, ∆3 = 0 and D4 = 0.

Note 13.12.1: Of these four conditions only two are independent since if two planes have two points in common then they show the line joining these two points should also have in common. It can be proved if any two of these conditions are satisfied, then the other two will also satisfy.

Aliter: The equations of the line of intersection of (13.37) and (13.38) are given by a1d2 − b2 d1 a d −ad y− 2 1 1 2 a1b2 − a2 b1 a1b2 − a2 b1 z−0 = = b1c2 − b2 c1 a1c1 − a2 c1 a1b2 − a2 b1

x−

(13.41)

If the planes (13.37), (13.38) and (13.39) intersect in a plane then the conditions are (i) the line (13.41) must be parallel to the plane (13.39) and (ii) the bd −b d a d −a d  point  1 2 2 1 , 2 1 1 2 , 0 must lie on the plane given by (13.39). The  a1b2 − a2 b1 a1b2 − a2 b1  conditions for (13.37) is a3 (b1c2 − b2 c1 ) + b3 ( a2 c1 − a1c2 ) + c3 ( a1b2 − a2 b1 ) = 0.



a1 (i.e.)  a2 a3

b1 b2 b3

c1 c2 = 0 (i.e.) D4 = 0 c3

The condition (ii) is given by (i.e.)

bd −b d  a d −ad  a3  1 2 2 1  + b3  2 1 1 2  + d3 = 0  a1b2 − a2 b1   a1b2 − a2 b1  a3(b1d2 - b2d1) + b3(a2d1 - a1d1 - a1d2) + d3(a1b2 - a2b1) = 0

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13-16

Analytical Geometry

a1 b1 d1 (i.e.)  a2 b2 d2 = 0 (i.e.) ∆3 = 0 a3 b3 d3 Therefore, the conditions for planes to intersect in a line are ∆3 = 0 and ∆4 = 0. (ii) Condition for the plane to intersect at a point: Solving equations (13.37), (13.38) and (13.39), we get

x y z 1 = = = ∆1 ∆ 2 ∆ 3 ∆ 4

∴x =

∆ ∆1 ∆ ,y = 2 ,z = 3 ∆4 ∆4 ∆4

If the planes intersect at a point then ∆4 ≠ 0. Hence the condition for a plane to intersect at a point is ∆4 ≠ 0.

Aliter: If the planes meet at a point then the line of intersection of any two planes is non-parallel to the third plane. Let l, m, n be the direction cosines of the intersection of planes (13.37) and (13.38). Then a1l + b1m + c1n = 0 a2l + b2m + c2n = 0



Solving the two equations for l, m, n we get, l m n = = b1c2 − b2 c1 a2 c1 − a1c2 a1b2 − a2 b1 Therefore, the direction ratios of the lines are b1c2 − b2 c1 , a2 c1 − a1c2 , a1b2 − a2 b1. Also the line of intersection will not be parallel to the third plane. \ a3 (b1c2 − b2 c1 ) + b3 ( a2 c1 − a1c2 ) + c3 (a1b2 − a2 b1 ) ≠ 0 a1 (i.e.)  a2 a3 This is the required condition.

b1 b2 b3

c1 c2 ≠ 0 or ∆4 ≠ 0 c3

13.13  Conditions for three given planes to form a triangular prism The line of intersection of planes (13.37) and (13.38) is given by a d − a2 d1 bd −b d x− 1 2 2 1 y− 1 2 a1b2 − a2 b1 a1b2 − a2 b1 z−0 = = b1c2 − b2 c1 a2 c1 − a1c2 a1b2 − a2 b1

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Straight Line

13-17

The three planes form a triangular prism if the line is parallel to the third plane. The conditions for this are the line is normal to the plane (13.39) and bd −b d a d −a d  the point  1 2 2 1 , 2 1 1 2 , 0 does not lie on the plane (13.39).  a1b2 − a2 b1 a1b2 − a2 b1  \ a3 (b1c2 − b2 c1 ) + b3 (c1a2 − c2 a1 ) + c3 (a1b2 − a2 b1 ) = 0

and



bd −b d  a d −ad  a3  1 2 2 1  + b3  2 1 1 2  + d3 ≠ 0  a1b2 − a2 b1   a1b2 − a2 b1 



a1 a2 a3

b1 b2 b3

c1 c2 = 0 and c3

a1 a2 a3

b1 b2 b3

d1 d2 ≠ 0 d3

(i.e.)  ∆4 = 0 and ∆3 ≠ 0. These are the required conditions.

ILLUSTRATIVE EXAMPLES Example 13.1 Find the equation of the line joining the points (2, 3, 5) and (-1, 2, -4).

Solution The direction ratios of the line are 2 + 1, 3 - 2, 5 + 4 (i.e.) 3, 1, 9. Therefore, x −2 y −3 z −5 = = . the equations of the line are 3 1 9

Example 13.2 Find the equation of the line passing through the point (3, 2, -6) and perpendicular to the plane 3x - y - 2z + 2 = 0.

Solution The direction ratios of the line are the direction ratios of the normal to the plane. Therefore, the direction ratios of the line are 3, -1, -2. Given that (4, 2, -6) is a point on the plane. x −3 y −2 z +6 = = . Therefore, the equations of the line are 3 −1 −2

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13-18

Analytical Geometry

Example 13.3 Find the equations of the line passing through the point (1, 2, 3) and perpendicular to the planes x - 2y - z + 5 = 0 and x + y + 3z + 6 = 0.

Solution Let l, m, n be the direction ratios of the line of intersection of the planes x - 2y z + 5 = 0 and x + y + 3z + 6 = 0. Then    l - 2m - n = 0 and     l + m + 3n = 0 l m n = = ∴ −6 + 1 −1 − 3 1 + 2 l m n = = . (i.e.)  −5 −4 3 Since the line also passes through the point (-1, 2, 3), its equations is x −1 y − 2 z − 3 = = . 5 4 −3

Example 13.4 Express the symmetrical form of the equations of the line x + 2y + z - 3 = 0, 6x + 8y + 3z - 13 = 0.

Solution To express the equations of a line in symmetrical form we have to find (i) the direction ratios of the line and (ii) a point on the line. Let l, m, n be the direction ratios of line. Then l + 2m + n = 0 and 6l + 8m + 3n = 0. l m n = = 6 − 8 6 − 3 8 − 12 l m n l m n = = or = = (i.e.)  −2 3 −4 2 −3 4

\

Let us find the point where the line meets the xy-plane (i.e.) z = 0. ∴ x + 2y − 3 = 0 6 x + 8 y − 13 = 0



x y 1 = = or −26 + 24 −18 + 13 8 − 12

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x y 1 = = 2 5 4

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Straight Line

13-19

1 5 ,y= ,z=0 2 4 5 1 x− y− 4 = z −0. 2= Therefore, the equations of the line are 4 2 −3 ∴x =



Example 13.5 Find the perpendicular distance from the point (1, 3, -1) to the line x − 13 y + 8 z − 31 = = . 5 −8 1

Solution The equations of the line are x − 13 y + 8 z − 31 = =r = 5 −8 1



P(1, 3, −1)

R (13, −8, 31)

Q

Any point on this line are (5r + 13, −8r − 8, r + 31). Draw PQ perpendicular to the plane. The direction ratios of the line are (5r + 12, −8r − 11, r + 32). Since the line PQ is perpendicular to QR, we have

5(5r + 12) − 8( −8r − 11) + 1( r + 32) = 0



25r + 60 + 64 r + 88 + r + 32 = 0 or 90 r + 180 = 0 ⇒ r = −2



Q is the point (3, 8, 29) and P is (1, 3, -1)

∴ PQ 2 = (3 − 1)2 + (8 − 3)2 + (29 + 1)2 = 4 + 25 + 900 = 929



∴ PQ 2 = 929 units

Example 13.6 Find the equation of plane passing through the line x y z parallel to the line = = . 1 2 3

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 19

x −1 y +1 z − 3 = = and 2 −1 4

4/22/2013 8:22:40 PM

13-20

Analytical Geometry

Solution Any plane containing the line x −1 y +1 z − 3 = = 2 −1 4

(13.42)

A( x − 1) + B( y + 1) + C ( z − 3) = 0

(13.43)

2A – B + 4C = 0

(13.44)

is where

Also the line is parallel to the plane x y z = = 1 2 3



∴ A + 2B + 3C = 0



(i.e.)  11x + 2y - 5z + 6 = 0

(13.45)

(13.46) A B C = = Solving for A, B and C from (13.44) and (13.46), we get −3 − 8 4 − 6 4 + 1 A B C or = = 11 2 −5 Therefore, the equation of the required plane is 11(x - 1) + 2(y + 1) 5(z - 3) = 0.

Example 13.7 Find the image of the point (2, 3, 5) on the plane 2x + y - z + 2 = 0.

Solution Let Q be the image of the point P(2, 3, 5) on the plane 2x + y – z + 2 = 0. The x −2 y −3 z −5 = = = r. equation of the line PQ is 2 1 −1 Any point on this line is (2r + 2, r + 3, -r + 5). When the line meets the plane, this point lies on the plane 2x + y - z + 2 = 0. P(2, 3, 5)

R

Q (x1, y1, z1)

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Straight Line

13-21

∴ 2(2r + 2) + r + 3 - (-r + 5) + 2 = 0 −2 Or r + 4 + 3 + r + 3 + r – 5 + 2 = 0 or 6r + 4 = 0 or r = 3 −2 2   −4 Therefore, the coordinates of R are  + 2, + 3, + 5 .  3 3 3   2 7 17  (i.e.)   , ,  . 3 3 3

If Q is the point (x1, y1, z1) then

2 + x1 2 3 + y1 7 5 + z1 17 = , = , = 2 3 2 3 2 3 −2 5 19 , y1 = , z1 = . ∴ x1 = 3 3 3  −2 5 19  ∴ Q is  , ,   3 3 3

Example 13.8 Find the image of the line

x −1 y − 3 z − 4 = = = r in the plane 2x - y + z + 3 = 0. 3 5 2

Solution The equations of the line are x −1 y − 3 z − 4 = = = r 3 5 2 Any point on this line is (3r + 1, 5r + 3, 2r + 4). As this point lies on the plane,



(13.47)

 2x - y + z + 3 = 0 we have 2(3r + 1) – (5r + 3) + (2r + 4) + 3 = 0.   (i.e.)  3r + 6 = 0 or r = -2.

(13.48)

P(1, 3, 4)

R

L

Q (x1, y1, z1)

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13-22

Analytical Geometry

Hence the coordinates of R are (-5, -7, 0). The equations of the line PL perpendicular to the plane are x −1 y − 3 z − 4 = = = r1 2 −1 1 Any point on this line is (2r1 + 1, − r1 + 3, r1 + 4). If this point lies on the plane (13.48), we get 2(2r1 + 1) − ( − r1 + 3) + ( r1 + 4) + 3 = 0.





(i.e.)  6r1 + 6 = 0 or r1 = -1.

Therefore, the coordinates of L where this line meets the plane (13.47) are (-1, 4, 3). If Q (x1, y1, z1) is the image of P in the plane 1 + x1 3 + y1 z +4 = −1, = 4, 1 =3 2 2 2 or x1 = - 3, y1 = 5, z1 = 2. ∴ Q is (-3, 5, 2). x+5 y+7 z = = . Hence the equations of the reflection line RQ are 1 6 1

Example 13.9 Find the equation of the straight lines through the origin each of which interx −3 y −3 z = = and are inclined at an angle of 60° to it. sects the straight line 2 1 1

Solution The equations of the line PQ are x −3 y −3 z = = =r 2 1 1



O

60° P

60° Q

The point P on this line is P(2r + 3, r + 3, r). p The direction ratios of OP are 2r + 3, r + 3, r. Since POQ = , 3 p 2(2r + 3) + 1( r + 3) + 1⋅ ( r ) cos = 3 (2r + 3)2 + ( r + 3)2 + r 2 ⋅ 4 + 1 + 1

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Straight Line



(i.e.) 

6r + 9 (6r + 18r + 18) 6 2

=

1 2



9(2r + 3)2 1 4 r 2 + 12r + 9 1 (i.e.)  36( r 2 + 3r + 3) = 2 ⇒ 4( r 2 + 3r + 3) = 4



(i.e.)  4r2 + 12r + 9 = r2 + 3r + 3



13-23

(i.e.)  3r2 + 9r + 6 = 0

or r2 + 3r + 2 = 0 or r = -1, -2. Therefore, the coordinates of P and Q are (1, -2, -1) and (-1, 1, -2). x y z = = and Hence the equations of the lines OP and OQ are 1 2 − 1 x y z = = . 1 1 2

Example 13.10 Find the coordinates of the point where the line given by x + 3y - z = 6, y - z = 4 cuts the plane 2x + 2y + z = 6.

Solution Let l, m, n be the direction cosines of the line x + 3y - z = 6, y - z = 4. Then

l + 3m - n = 0

m - n = 0. l m n ∴ = = Therefore, the direction ratios of the line are 2, -1, -1. When −2 1 1 the line meets the xy-plane whose equation is z = 0, we have x + 3y = 6, y = 4. Therefore, the point where the line meets xy-plane is (-6, 4, 0). x+6 y−4 z Therefore, the equations of the line are = = = r. 2 −1 −1 Any point on this line is (2r - 6, -r + 4, -r). This point lies on the plane 2x + 2y + z = 0.

 2(2r - 6) + 2(-r + 4) - r = 0 \ r - 4 = 0 or r = 4 Hence the required point is (2, 0, -4).

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4/22/2013 8:22:49 PM

13-24

Analytical Geometry

Example 13.11 Find the distance of the point (1, -2, 3) from the plane x - y + z = 5 measured y parallel to the line x = = z . 2 3 −6

Solution y The equations of the line through (1, -2, 3) and parallel to the line x = = z 2 3 −6 y +2 z −3 = = r . Any point on this line is (2r + 1, 3r - 2, are x − 1 = 2 3 −6 -6r + 3). If this point lies on the plane x - y + z = 5 then (2r + 1) - (3r - 2) + (-6r + 3) = 5. (i.e.)  -7r + 1 = 0  or  r = 1 . 7 Therefore, the point P is 9 , −11 , 15 . 7 7 7 Therefore, the distance between the points A (1, -2, 3) and 9 , −11 , 15 is 7 7 7 2 2 2 2 9 15 9 36 11 4 AP = 1 − + −2 + + 3− = + + = 1. 7 7 7 49 49 49

(

( ) (

) (

)

)

(

)

Example 13.12 Prove that the equation of the line through the points (a, b, c) and (a′, b′, c′) passes through the origin if aa′ + bb′ + cc′ = pp′ where p and p′ are the distances of the points from the origin.

Solution The equations of the line through (a, b, c) and (a′, b′, c′) are

x−a = y−b = z −c a′ − a b′ − b c ′ − c

If this passes through the origin then − a = −b = −c ′ a′ − a b′ − b c ′ ′ ′ − − − a a b b c c = =   or  a′ = b′ = c′ (i.e.)  c a b a b c ∴ ab′ − a′b = 0   bc′ − b′c = 0  ca′ − a′a = 0 

(13.49)

Let p and p′ be the distances of the points (a, b, c) and (a′, b′, c′) from the origin.

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4/22/2013 8:22:52 PM

Straight Line

13-25

 \ p2 = a2 + b2 + c2 p′2 = a′2 + b′2 + c′2  p2p′2 = (a2 + b2 + c2) (a′2 + b′2 + c′2) By Lagrange’s identity,

(a2 + b2 + c2) (a′2 + b′2 + c′2) - (aa′ + bb′ + cc′)2



= (ab′ - a′b)2 + (bc′ - b′c)2 + (ca′ - c′a)2 = 0 using (3).



p2p′2 – (aa′ + bb′ + cc′)2 = 0  or  p2p′2 = (aa′ + bb′ + cc′)2



\ pp′ = aa′ + bb′ + cc′

Example 13.13 If from the point P(x, y, z), PM is drawn perpendicular to the line x = y = z and is produced to Q such that PM = MQ then show that l m n x + x1 y + y1 z + z1 2(lx + my + nz ) . = = = 2 l m n l + m 2 + n2

Solution y The equation of the line OA is x = = z = r . l m n Any point on this line is (lr, mr, nr). P (x, y, z)

O

M

A

Q (x1, y1, z1)

If M is this point then   lr =

x + x1 y + y1 z + z1 x + x1 y + y1 z + z1 , mr = , nr = or = = = 2r (13.50) 2 2 2 l m n

The direction ratios of the line MP are x - lr, y - mr, z - nr. Since MP is perpendicular to OA,

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 25

4/22/2013 8:22:54 PM

13-26

Analytical Geometry

l(x - lr) + m(y - mr) + n(z - nr) = 0  or   lx + my + nz = r(l2 + m2 + n2) lx + my + nz r= 2 l + m 2 + n2 x + x1 y + y1 z + z1 2(lx + my + nz ) From (13.50), we get . = = = 2 l m z l + m 2 + n2

Example 13.14 Reduce the equations of the lines x = ay + b, z = cy + d to symmetrical form and hence find the condition that the line be perpendicular to the line whose equations are x = a′y + b′, z = c′y + d′.

Solution The line x = ay + b, z = cy + d (13.51) can be expressed in the symmetrical form as x−b = y = z −d (13.52) 1 c a The line x = a′y + b′, z = cy′ + d′ (13.53) in symmetrical form is x − b′ = y = z − d ′  (13.54) 1 c′ a′ If the lines (13.51) and (13.52) are perpendicular then aa′ + bb′ + cc′ = 0. This is the required condition.

Example 13.15 Find the equation of the line passing through G perpendicular to the plane XYZ represented by the equation lx + my + nz = p where l2 + m2 + n2 = 1 and calculate the distance of G from the plane.

Solution The equation of the plane xyz is lx + my + nz = p (13.55) where l2 + m2 + n2 =1. When this plane meets the x-axis, y = 0 and z = 0. p ∴x = l p  p   p  Hence X is the point  , 0, 0  . Similarly, Y is  0, , 0  and Z is  0, 0,  . n  m   l   p p p The centroid of DXYZ is  , , .  3l 3m 3n 

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4/22/2013 8:22:56 PM

Straight Line

13-27

The equation of the line through G perpendicular to the plane XYZ is p p p x− y− z− 3l = 3m = 3n = r . l m n When this line meets the YOZ plane, x = 0. (13.56) p Then − 2 = r 3l Here, r is the distance of G from the plane (13.55) since p and l2 are positive, r = GA p or GA = 2 . 3l GB =

Similarly,

p p , GC = 2 2 3m 3n

∴ 1 + 1 + 1 = 3 ( l 2 + m 2 + n2 ) = 3 . GA GB GC p p



Example 13.16 Find the perpendicular distance of angular points of a cube from a diagonal which does not pass through the angular point.

Solution z C′

O (0, 0, 0)

A′

B′ O′ (0, a, 0)

O

A

B

y

L

B′

C

x

Let a be the side of the cube. BB′ is a diagonal of the cube not passing through O. The direction ratios of BB′ are a, -a, a. (i.e.) 1, -1, 1. The direction cosines of BB′ are 1 , −1 , 1 . The projections of OB′ on BB′ 3 3 3

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 27

4/22/2013 8:22:58 PM

13-28

Analytical Geometry



= LB′ = (x1 - x2)l + (y1 -y2)m + (z1 - z2)n



  = ( a − 0) 1 + ( 0 − a)  − 1  + ( 0 − a) 1 = a 3 3 3 3 

2 OB = 0 + a 2 + 0 = a   OL′2 = OB′2 - LB′2 = a2 - a 3 2 2 a 2 ′  OL = 3



∴ OL = 2 a = 6 a 3 3



Example 13.17 Prove that the equations of the line through the point (a, b, g  ) and at x − x1 y y = = z, x = = z are right angles to the lines x = m1n2 − m2 n1 l1 m1 n1 l2 m2 n2 y − y1 z − z1 . = n1l2 − n2 l1 l1m2 − l2 m1

Solution Let l, m, n be the direction cosines of the line perpendicular to the two given lines. Then we have ll1 + mm1 + nn1 = 0

ll2 + mm2 + nn2 = 0



l m n = = m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1

Therefore, the direction ratios of the line are m1n2 - m2n1, n1l2 - n2l1, l1m2 - l2m1. The line also passes through the point (x1, y1, z1). Its equations are

x − x1 y − y1 z − z1 . = = m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1

Exercises 1 1.

Show that the line z + 4 = 0.

x −1 = y −1 = z is parallel to the plane 2x + 3y 2 −1 1

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4/22/2013 8:23:01 PM

Straight Line

13-29

x +1 = y − 3 = z + 2 and the −3 2 1

2.

Find the equation of the plane through the line



point (0, 7, -7). Show further the plane contains the line x = y − 7 = z + 7 . 1 −3 2 Ans.: x + y + z = 0

3.  4.  5.

 6.



Find the equation of the plane which passes through the line 3x + 5y + 7z - 5 = 0 = x + y + z - 3 and parallel to the line 4x + y + z = 0 = 2x - 3y - 5z. Ans.: 2x + 4y + y + 6z = 2 Find the equations of the line through the point (1, 0, 7) which intersect each of the lines x = y = z, x - 1 = y - 5 = z + 1. 2 Ans.: 7x - 6y - z = 0, 9x - 7y - z - 2 = 0 Find the equation of the plane which passes through the point (5, 1, 2) and is x − 2 = y − 4 = z − 5. Find also the coordinates of perpendicular to the line 1 2 2 the point in which this line cuts the plane. Ans.: x - 2y - 2z - 1 = 0; (1, 2, 3) Find the equation of the plane through (1, 1, 2) and (2, 10, -1) and x−4 = y −2 = z −6 . perpendicular to the straight line 3 1 7 Ans.: 3x - y -7z + 2 = 0

7.

Find the projection of the line 3x - y + 2z = 1, x + 2y - z = 2 on the plane 3x + 2y + z = 0.

 8.

Ans.: 3x + 2y + z = 0, 3x - 8y + 7z + 4 = 0 Find the projection of the line x = 3 - 6t, y = 2t, z = 3 + 2t in the plane 3x + x − 9 = y + 2 = z −1 4y - 5z - 26 = 0. Ans.: 18 −1 −10 Find the equation of the plane which contains the line and is perpendicular to the plane x + 2y + z = 12. Ans.: 9x - 2y - 5z + 4 = 0

9. 10. 11.

 12.

Find the equation of the plane which passes through the z-axis and is y = z − 3 . Ans.: xcosa + ysina = 0 perpendicular to the line x − 1 = cos a sin a 0 Find the equations of two planes through the origin which are parallel to the x −1 = y + 3 = z +1 5 and distant from it. Show also that the two planes line 2 −1 −2 3 are perpendicular. Ans.: x + 2y - 2z = 0, 2x + 2y + z = 0 Find the equations to the line of the greatest slope through the point (1, 2, -1) in the plane x - 2y + 3z = 0 assuming that the axes are so placed that the plane x − 7 = y − 2 = z +1 2x + 3y - 4z = 0 is horizontal. Ans.: 2z 5 −4

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4/22/2013 8:23:03 PM

13-30

13.

 14.

 15.

16.

17.

Analytical Geometry

y x = z as vertical, find the equation of the line of the Assuming the line = 4 −3 7 greatest slope in the plane 2x + y - 5z = 12 and passing through the point (2, 3, -1). x − 2 = y − 3 = z +1 Ans.: 3 −1 1 y x = z is vertical. Find the With the given axes rectangular the line = 2 −3 1 direction cosines of the line of the greatest slope in the plane 3x - 2y + z = 0 and the angle of this line makes with the horizontal plane. 16 , −1 ;sin −1 −9 Ans.: 11 , 1453 378 378 378 y y y Show that the lines x = = z ; x = = z ; x = = z will be coplanar a b g aa bb cg l m n if l (b − c) + m(c − a) + n( a − b) = 0. a b g x y z Show that the equation of the plane through the line = = and which l m n x= y=z x y is perpendicular to the plane containing the lines and = = z l n l n l m is S(m - n)x = 0.

( )

Show that the line

y ax by g z x y z ; = = and x = = z will lie in = = al bm g n l m n l m n

a plane if a = b or b = g or g = a. 18.

Find the equation of the plane passing through the line and perpendicular to the plane x + 2y + z = 12.

Ans.: 9x - 2y + 5z + 4 = 0

 19.  20.

 21.

22.

x −1 = y +1 = z − 3 2 −1 4

Find the equations of the line through (3, 4, 0) and perpendicular to the plane 2x + 4y + 7z = 8. x−3 = y−4 = z Ans.: 2 4 7 Find the equation of the plane passing through the line x y z are parallel to the line = = . 1 2 3

x −1 = y +1 = z − 3 2 −1 4

Ans.: 4y - 3z + 1 = 0, 2x - 7z + 1 = 0, 3x - 2y + 1 = 0. Show that the equation of the planes through the line which bisect the angle y x y z x = z (where l, m, n and l′, m′, n′ are between the lines = = ; = l m n l ′ m ′ n′ direction cosines) and perpendicular to the plane containing them are (l + l′)x + (m + m′)y + (n + n′)z = 0. x −1 = y − 2 = z − 3 and Find the equation of the plane through the line 2 3 4 parallel to the coordinate planes.



M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 30

Ans.: x cosq + y sinq = 0

4/22/2013 8:23:07 PM

Straight Line

2 3.

2 4.

13-31

Prove that the plane through the point (a, b, g ) and the line x = py + q = zx + r x py + q rz + s is given by a pb + q rg + s = 0. 1 1 1 y The line L is given by x − 1 = = z + 2 . Find the direction cosines of the 2 2 −1 projections of L on the plane 2x + y -3z = 4 and the equation of the plane through L parallel to the line 2x + 5y + 3z = 4, x - y - 5z = 6. Ans.:



2 , −1 , 1 ; 3 x + 4 y − 2 z = 7 6 6 6

Exercises 2 1.

Find the equation of the line joining the points



(i) (2, 3, 5) and (-1, 2, -4)



(ii) (1, -1, 3) and (3, 3, 1)

Ans.: (i) x − 2 = y − 3 = z ; (ii) x − 1 = y + 1 = z − 3 3 1 9 2 4 −2

 2.  3.  4.  5.  6.

Find the equations of the line passing through the point (3, 2, -8) and is perpendicular to the plane 3x - y - 2z + 2 = 0. x −3 = y −2 = z+8 Ans.: 3 −1 −2 Find the equations of the line passing through the point (3, 1, -6) and parallel to each of the planes x + y + 2z - 4 = 0 and 2x - 3y + z + 5 = 0. x − 3 = y −1 = z + 6 Ans.: 3 3 −5 Find the equations of the line through the point (1, 2, 3) and parallel to the line of intersection of the planes x - 2y - z + 5 = 0, x + y + 3z - 6 = 0. x − 2 = y − 3 = z −1 Ans.: 5 4 −3 x −1 = y − 2 = z + 3 Find the point at which the line meets the plane 2x + 2 3 4 4y - z + 1 = 0. 10 , −3 , 5 Ans.: 3 2 3

(

Find the coordinates of the point at which the line the plane 2x + 3y + z = 0.



M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 31

)

x = y −1 = z + 2 meets 1 2 3 Ans.:

( 11−1 , 119 , 1125 ) 4/22/2013 8:23:10 PM

13-32

Analytical Geometry

7.

Prove that the equations of the normal to the plane ax + by + cz + d = 0 x −a = y − b = z −g . through the point (a, b, g ) are a b c

8.

Express in symmetrical form the following lines: (i) x + 2y + z = 3, 6x + 8y + 3z = 13 (ii) x - 2y + 3z - 4 = 0, 2x - 3y + 4z - 5 = 0 (iii) x + 3y - z - 15 = 0, 5x - 2y + 4z + 8 = 0 x − 2 = y +1 = z − 3 Ans.: (i) 2 −3 4 x+2 = y+3 = z (ii) 1 2 1 x − 2 = y −1 = z (iii) 2 −1 −3

9.

Prove that the lines 3x + 2y + z - 5 = 0, x + y - 2z - 3 = 0 and 8x - 4y - 4z = 0, 7x + 10y - 8z = 0 are at right angles.

10.

Prove that the lines x - 4y + 2z = 0, 4x - y - 3z = 0 and x + 3y - 5z + 9 = 0, 7x - 5y - z + 7 = 0 are parallel.

11.

Find the point at which the perpendicular from the origin on the line joining the points (-9, 4, 5) and (11, 0, -1) meets it. Ans.: (1, 2, 2).

 12.

Prove that the lines 2x + 3y - 4z = 0, 3x - 4y + 7 = 0 and 5x - y - 3z + 12 = 0, x - 7y + 5z - 6 = 0 are parallel.

13.

Find the perpendicular from the point (1, 3, 9) to the line x − 13 = y + 8 = z − 31 . 5 −8 1

 14.  15.  16.



Ans.: 21

Find the distance of the point (-1, -5, -10) from the point of intersection of x − 2 = y +1 = z − 2 and the plane x - y + z = 5. the line 3 4 12 Ans.: 13 Find the length of the perpendicular from the point (5, 4, -1) to the line x −1 = y = z . 2 9 5 2109 Ans.: 110 Find the foot of the perpendicular from the point (-1, 11, 5) to the line x −2 = y+3 = z. 3 2 2 Ans.: 17 , 7 , 58 191 17 17

(

)

17.

Obtain the coordinates of the foot of the perpendicular from the origin on the line joining the points (-9, 4, 5) and (11, 0, -1).

18. 

Find the image of the point (4, 5, -2) in the plane x - y + 3z - 4 = 0. Ans.: (6, 3, 4)

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Straight Line

13-33

19. 

Find the image of the point (1, 3, 4) in the plane 2x - y + z + 3 = 0. Ans.: (1, 0, 7)

20.

Find the image of the point (2, 3, 5) in the plane 2x + y - z + 2 = 0.



Ans.:



( −32 , 53 , 193 )

Find the image of the point (p, q, r) in the plane 2x + y + z = 6 and hence find x −1 = y − 2 = z − 3 . the image of the line 2 1 −4 Ans.:  12 − p − 2q − 2r , 6 − 2 p + 2q − r , 6 − 2 p − q + 2r , 3 x − 1 = 3 y − 5 = 3 z − 8  3 3 3 4 2 −13   21.

22.

 23.

 24.

25. 

Find the coordinates of the foot of the perpendicular from (1, 0, 2) to the line x + 1 = y − 2 = z + 1 . Also find the length of the perpendicular. 3 −3 −1 8 , 11 , −28 ; 11 38 Ans.: 19 19 19 19

(

)

Find the equation in symmetrical form of the projection of the line x −1 = y +1 = z − 3 on the plane x + 2y + z = 12. 2 −1 4 5 x − 4 = 5 y − 28 = z Ans.: 4 −7 2 x −1 = y = z − 3 meets the plane 2x + 3 −2 1 35y – 39z + 12 = 0 is equidistant from the planes 12x - 15y + 16z = 28 and 6x + 6y – 7z = 8. x −1 = y +1 = z on the Find the equation of the projection of the straight line 1 2 3 plane x + y + 2z = 5 in symmetrical form. x 3 y − 5 = 3z − 5 Ans.: = 1 −3 0 Prove that the point which the line

26.

Prove that two lines in which the planes 3x - 7y - 5z = 1 and 5x - 13y + 3z + 2 = 0 cut the plane 8x – 11y + 2z = 0 include a right angle.

27.

Reduce to symmetrical form the line given by the equations x + y + z + 1 = 0, 4x + y - 2z + 2 = 0. Hence find the equation of the plane through (1, 1, 1) and perpendicular to the given line. 3x + 1 = 3 y + 2 = z ; x − 2 y + z = 0 Ans.: 3 −6 1

 28.  29.

Show that the line x + 2y - z - 3 = 0, x + 3y - z - 4 = 0 is parallel to the xz-plane and find the coordinates of the point where it meets yz-plane. Ans.: (0, 1, -1) Find the angle between the lines x - 2y + z = 0, x + y - z - 3 = 0, and x + 2y + z - 5 = 0, 8x + 12y + 5z = 0.



M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 33

 8  Ans.: cos-1  406 

4/22/2013 8:23:16 PM

13-34

Analytical Geometry

x −1 = y +1 = z − 3 2 −1 4

30.

Find the equation of the plane passing through the line x y z and parallel to the line = = . 1 2 3



Ans.: 11x + 2y - 5z + 6 = 0 x + y + z =1 meets the axes in A, B and C. Find the coordinates of The plane a b c the orthocentre of the DABC. −1 −1  a −1  , b , c Ans.:   ∑ a −2 ∑ a −2 ∑ a −2 

31.

 32.



x = y = z. Through a point P(1, 2, 3), PN is 1 −2 3 drawn perpendicular to AB and PQ is drawn parallel to the plane 2x + 3y + 4z = 0 to meet AB in Q. Find the equations of PN and PQ and the coordinates of N and Q. x − 1 = y − 2 = z − 3 ; x − 1 = y − 2 = z − 3 ; 5 , − 5, 15 Ans.: 1 5 3 3 −14 9 2 2

The equation to a line AB are

(

)

ILLUSTRATIVE EXAMPLES (COPLANAR LINES AND SHORTEST DISTANCE) Example 13.18

y + 10 z − 1 y +1 z − 4 Prove that the lines x + 1 = = = are coplaand x + 3 = 8 7 1 2 −3 −4 nar and find the equation of the plane continuing these two lines.

Solution (-1, -10, 1) is a point on the first line and -3, 8, 2 are the direction ratios of the first line. (-3, -1, 4) is a point on the second line and -4, 7, 1 are the direction ratios of the second line. If the lines are coplanar then x2 − x1 y2 − y1 z2 − z1 l1 m1 n1 = 0. l2 m2 n2

Now

2 −9 −3 −3 8 2 = 2(8 − 14) + 9( −3 + 8) − 3( −21 + 32) −4 7 1

 = −12 + 45 − 33 = 0

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Straight Line

13-35

Therefore, the two lines are coplanar. The equation of the plane containing x + 1 y + 10 z − 1 8 2 = 0. the lines is −3 −4 7 1 (i.e.)  (x + 1) (8 - 14) - (y + 10) (-3 + 8) + (z - 1) (-21 + 32) = 0

(i.e.)  -6x - 6 - 5y - 50 + 11z - 11 = 0



6x + 5y - 11z + 67 = 0

Example 13.19

y −3 z +2 y−7 z+7 and x = = = intersect. Show that the lines x + 1 = 2 1 −3 −3 1 2 Find the point of intersection and the equation of the plane containing these two lines.

Solution The two given lines are

x +1 = y − 3 = z + 2 = r −3 2 1

(13.57)

x = y−7 = z+7 = r 1 1 2 −3

(13.58)



Any point on the first line is (-3r - 1, 2r + 3, r - 2). Any point on the second line is (r1, -3r1 + 7, 2r1 - 7). If the two lines intersect then the two points are one and the same.

- 3r - 1 = r1        3r + r1 = -1

(13.59)



2r + 3 = -3r1 + 7  or  2r + 3r1 = 4

(13.60)



r – 2 = 2r1 - 7     r - 2r1 = -5



2r + 3r1 = 4



2r - 4r1 = -10



(13.61)

7r1 = 14 r1 = 2, r = -1

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 35

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13-36

Analytical Geometry

Solving (13.60) and (13.61), we get r = -1 and r1 = 2. These values satisfy equation (13.59). The point of intersection is (2, 1, -3). The equation of the plane containing the two lines is x +1 y − 3 z + 2 −3 +2 1 = 0 (i.e.) ( x + 1)( 4 + 3) − ( y − 3)( −6 − 1) + ( z + 2)(9 − 2) = 0 1 −3 2

(i.e.)  7 x + 7 + 7 y − 21 + 7 z + 14 = 0



(i.e.)  7 x + 7 y + 7 z = 0 ⇒ x + y + z = 0

Example 13.20

y +1 z +1 and x + 2y + 3z - 8 = 0, 2x + 3y + 4z = Show that the lines x + 1 = 1 2 3 11 = 0 are coplanar. Find the equation of the plane containing these two lines.

Solution The two lines are

x +1 = y +1 = z +1 = r 1 2 3 x + 2y + 3z - 8 = 0

 2x + 3y + 4z - 11 = 0

(13.62)

(13.63)

Any plane containing the second line is

x + 2y + 3z - 8 + l (2x + 3y + 4z - 11) = 0

(13.64)

If the line given by (13.62) lies on this plane then the point (-1, -1, -1) also lies on the plane.

-1 -2 -3 -8 + l (-2 - 3 - 4 -11) = 0

⇒ -14 + l (-20) = 0 ∴ l = −7 10 The equation of the plane (13.64) is x + 2 y + 3 z − 8 − 7 (2 x + 3 y + 4 z − 11) = 0 10





10x + 20y + 30z - 80 - 14x - 21y - 28z + 77 = 0

or

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 36

4x + y - 2z + 3 = 0

(13.65)

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Straight Line

13-37

Also the normal to this plane should be perpendicular to the line (13.62). The direction ratios of the normal to the plane are 4, 1, -2. The direction ratios of the line (13.62) are 1, 2, 3. Also ll1 + mm1 + nn1 = 4 + 2 - 6 = 0 which is true. Hence, the plane containing the two given lines is 4x + y - 2z + 3 = 0. Any point on the first line is (r - 1, 2r - 1, 3r - 1). If the two given lines intersect at this point then it should lie on the second line and hence on the plane x + 2y + 3z - 8 = 0. \ r - 1 + 2(2r - 1) + 3(3r - 1) - 8 = 0  (i.e.)  14r - 14 = 0 or r = 1 Therefore, the point of intersection of the two given lines is (0, 1, 2).

Example 13.21 Show that the lines x + 2y + 3z - 4 = 0, 2x + 3y + 4z - 5 = 0 and 2x + 3y + 3z - 5 = 0, 3x - 2y + 4z - 6 = 0 are coplanar and find the equation of the plane containing the two lines.

Solution Let us express the first line in symmetrical form. Let l, m, n be the direction cosines of the first line. Then this line is perpendicular to the normals of the planes x + 2y + 3z - 4 = 0 and 2x + 3y + 4z - 5 = 0. \ l + 2m + 3n = 0 Solving, we get

2l + 3m + 4n = 0 l m n l = m = n or −1 = 2 = −1 8−9 6− 4 3− 4

Therefore, the direction ratios of the first line are 1, -2, 1. To find a point on the first line let us find where this line meets the XOY plane (i.e.) z = 0. x + 2y = 4 2x + 3y = 5 Solving these two equations we get the point as (-2, 3, 0). Therefore, the equations of the first line are x+2 = y −3 = z 1 1 −2 Any plane containing the second line is



2x - 3y + 3z - 5 + l (3x - 2y + 4z - 6) = 0

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 37

(13.66)

(13.67)

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13-38

Analytical Geometry

If the plane contains the second line then the point (-2, 3, 0) should lie on the plane (13.67). \ -4 - 9 - 5 + l (-6 - 6 - 6) = 0 or -18 - 18l  = 0 l = -1

or

Hence the equations of the plane (13.67) becomes x + y - 1 = 0



(13.68)

Also it should satisfy the condition. That the normal to the plane should be perpendicular to the line (13.66). The direction ratios of the normal to the plane (13.68) are 1, 1, 1. The direction ratios of the line are 1, -2, 1. Also 1 - 2 + 1 = 0 which is satisfied. Hence the equation of the required plane is x + y + z - 1 = 0.

Example 13.22 Prove that the lines x = ay + b = cz + d and x = a y + b = g  z + d are coplanar if (r - c) (a b - bd) - (a - a) (a d - dg ) = 0.

Solution First let us express the given lines in symmetrical form. The two given lines b z+d y+ y+ b z+ d g a = a = c and x = . are x = ag g a ac c a 0 Then two lines are coplanar if ac ag

b−b a a c g

d −d c g a = 0. a

b     (i.e.)  −  b −  ( aca − aag ) +  d − d  ( acg − a cg ) = 0 a a c g 

 ba − a b   dg − ed aa (c − g ) +  (i.e.)  −   aa   cg

  cg ( a − a ) = 0

 (i.e.)  (g - c) (ab - ba ) - (ad - dg ) (a - a) = 0

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Straight Line

13-39

Example 13.23 Prove that the lines a1x + b1y + c1z + d1 = 0 = a2x + b2y + c2z + d2 and a3x + b3y + a1 a c3z + d3 = 0 = a4x + b4y + c4z + d4 are coplanar if 2 a3 a4

b1 b2 b3 b4

c1 c2 c3 c4

d1 d2 = 0. d3 d4

Solution Let the two lines intersect at (x1, y1, z1). Then (x1, y1, z1) should lie on the planes containing these lines. a1 x1 + b1 y1 + c1 z1 + d1 = 0 a2 x1 + b2 y1 + c2 z1 + d2 = 0



a3 x1 + b3 y1 + c3 z1 + d3 = 0 a4 x1 + b4 y1 + c4 z1 + d 4 = 0

Eliminating (x1, y1, z1) from the above equations we get a1 a2 a3 a4



b1 b2 b3 b4

c1 c2 c3 c4

d1 d2 = 0. d3 d4

This is the required condition.

Example 13.24 Find the shortest distance and the equation to the line of shortest distance y+4 z+3 y+5 z−2 . = and x − 21 = = between the two lines x + 7 = 6 −1 3 4 −2 −4

Solution The

two

given

lines

are

x+7 = y+4 = z+3 = r 3 4 −2

and

x − 21 = 6

y+5 z−2 = r1 . = −1 −4 The coordinates of any point P on the first line are (3r - 7, 4r - 4, -2r - 3). The coordinates of any point Q on the second line are (6r1 + 21, -4r1 -5, -r1 + 2).

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 39

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13-40

Analytical Geometry

The direction ratios of the line PQ are 3r - 6r1 - 28, 4r + 4r1 + 1, -2r + r1 - 5. If PQ is the line of the shortest distance then the two lines are perpendicular. The direction ratios of the two lines are 3, 4, -2 and 6, -4, -1. Then 3(3r 6r1 - 28) + 4(4r + 4r1 + 1) -2(-2r + r1 - 5) = 0 and 6(3r - 6r1 - 2r) - 4(4r + 4r1 + 1) - 1(-2r + r1 - 8) = 0 (i.e.)  29r - 4r1 - 90 = 0 4r - 53r1 - 167 = 0 Solving for r and r1, we get r = r1 = 1 ⇒ r = r1 = 1 −3042 4563 −1521 −2 3 −1 \ r = 2 and r1 = -3.



The coordinates of P and Q are given by P(-1, 4, -7) and Q(3, 7, 5). \ PQ2 = (3 + 1)2 + (7 - 4)2 + (5 + 7)2 = 16 + 9 + 144 = 169. \ PQ = 13 units y+4 z+7 = The equations of the line of the shortest distance are x + 1 = 3 +1 7 − 4 5 + 7 y + 4 = z + 7. (i.e.)  x + 1 = 4 3 12

Example 13.25 Show that the shortest distance between z-axis and the line of intersection of the plane 2x + 3y + z - 1 = 0 with 3x + 2y + z - 2 = 0 is

7 5 units. 25

Solution The equations of the plane containing the given line is

2x + 3y + 4z - 1 + l (3x + 2y + z - 2) = 0

(13.69)

The direction ratios of the normal to this plane are 2 + 3l, 3 + 2l, 4 + l. The direction ratios of the z-axis are 0, 0, 1. If z-axis is parallel to the line then 0(2 + 3l), 0(3 + 2l) + 1(4 + l) = 0. \ l = -4 Therefore, the equation of the plane (13.69) is 2x + 3y + 4z - 1 - 4(3x + 2y + z - 2) = 0

(i.e.)  -10x -5y + 7 = 0 or 10x + 5y - 7 = 0

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 40

(13.70)

4/22/2013 8:23:27 PM

13-41

Straight Line

Now the SD = the perpendicular distance from any point on the z-axis to the plane = perpendicular distance from (0, 0, 0) to the plane 7 5 units = 1 − 71 = 7 = 25 100 + 25 5 5

Example 13.26

y−7 y+9 z−2 = Find the points on the lines x − 6 = = z − 4 and x = 2 4 3 −1 −3 which are nearest to each other. Hence find the shortest distance between the lines and also its equation.

Solution The given lines are

x − 6 = y − 7 = z − 4 = r 3 −1 1 y+9 z−2   x = = = r1 −3 2 4



(13.71) (13.72)

Any point on the line (13.71) is P(3r + 6, - r + 7, r + 4). Any point on the line (13.72) is Q(- 3r1, 2r1 - 9, 4r1 + 2). The direction ratios of PQ are (3r + 3r1 + 6, - r - 2r1 + 16, r - 4r1 + 2). Since PQ is perpendicular to the two given lines.

3(3r + 3r1 + 6) - 1 (-r - 2r1 + 16) + 1(r - 4r1 + 2) = 0



-3(3r + 3r1 + 6) + 2(-r - 2r1 + 16) + 4(r - 4r1 + 2) = 0



11r + 7r1 + 4 = 0 (i.e.)  7r + 29r − 22 = 0 1

×7 ⇒

77r + 49r1 = −28

× 11 ⇒ 77r + 319r1 = 242

⇒ r1 = 1, r = - 1.



Therefore, the points P and Q are (3, 8, 3) and (-3, -7, 6). The SD is the distance PQ. \ PQ2 = (3 + 3)2 + (8 + 7)2 + (3 – 6)2 = 36 + 225 + 9 = 270.

PQ = 270 = 3 30 units

The direction ratios of PQ are 6, 15, -3 (i.e.) 2, 5, -1. P is (3, 8, 3). y −8 z −3 = . Therefore, the equations of the line of SD are x − 3 = 2 5 −1

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13-42

Analytical Geometry

Example 13.27

y + 9 z − 10 = and Find the shortest distance between the lines x − 8 = 3 −16 7 x − 15 = y − 29 = z − 15 . Find also the equation of the line of the shortest 3 8 −5 distance.

Solution Let l, m, n be the direction ratios of the line of the SD. Since it is perpendicular to both the lines 3l − 16m + 7n = 0 3l + 8m − 5n = 0 Solving for l, m, n, we get



l n = m = 80 − 56 21 + 15 24 + 48 (i.e.) l = m = n 24 36 72 (i.e.) l = m = n 2 3 6

The direction ratios of the line of SD are 2, 3, 6. The direction cosines of the line of SD are 2 , 3 , 6 . 7 7 7 The length of the line of the SD = ( x1 − x2 )l + ( y1 − y2 ) m + ( z1 − z2 )n m + ( z1 − z2 )n where (x 1, y 1, z 1) and (x 2, y 2, z 2) are the direction cosines of the line of SD. \ (x1, y1, z1) is (8, -9, 10) and (x2, y2, z2) is (15, 29, 15)

l, m, n are 2 , 3 , 6 . 7 7 7



∴ SD = (8 − 15) 2 + ( −9 − 29) 3 + (10 − 15) 6 = 158 units. 7 7 7 7

The equation of the plane containing the first line and the line of SD is x −8 3 2





y + 9 z − 10 −16 7 =0 3 6

(i.e.)  117 x + 4 y − 41z − 490 = 0

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Straight Line

13-43

The equation of the plane containing the second line and the line of SD is x − 18 3 2



y − 29 z − 15 −5 = 0. 8 3 6

(i.e.)  63x - 28y + 7z - 238 = 0 Therefore, the equations of the line of SD are 117x + 4y + 71z - 490 = 0, 63x - 28y + 7z - 238 = 0.



Example 13.28 y If 2d is the shortest distance between the lines x = 0, + z = 1 and b c y = 0, x − z = 1 then prove that 12 = 12 + 12 + 12 . a c d a b c

Solution The two given lines are y z + = 1 (13.73) b c y = 0, x − z = 1 (13.74) a c The equation of any plane containing the first line is y z + − 1 + l x = 0 (13.75) b c The equation of the second line in symmetrical form is x = y = z+c (13.76) c a 0 The plane given by equation (13.75) is parallel to the line (13.76). If l a + 1 ⋅ 0 + c ⋅ 1 = 0 ⇒ l = −1 c a b Hence from (13.75), the equation of the plane containing line (13.73) and y parallel to the plane (13.74) is x − − z + 1 = 0. a b c Then the SD between the given lines = the perpendicular distance from any point on the line (13.74) to the plane (13.75). (0, 0, -c) is a point on the line (13.76). x = 0,





∴ SD = 2d =

0 + 0 − ( −1) + 1 1 + 1 + 1 a2 b2 c2

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⇒1= d

1 + 1 + 1 a2 b2 c2

4/22/2013 8:23:35 PM

13-44

Analytical Geometry

1 = 1 + 1 + 1 d 2 a2 b2 c2

or

Example 13.29 Show that the shortest distance between any two opposite edges of the tetrahedron formed by the planes y + z = 0, z + x = 0, x + y = 0 and x + y + z = a is 2a and the three lines of the shortest distance intersect at the point 6 x = y = z = a.

Solution The equations of the edge determined by the planes y + z = 0, z + x = 0 is x= y= z (13.77) 1 1 −1 z

+

0

z+

=

x=

z

0

y

x+y+z =a

O =0 x+y

y

x

The equation of the opposite edges are x + y = 0, x + y + z = a

y = z − a (i.e.)  x = 1 −1 0

(13.78)

Let l, m, n be the direction cosines of the line of the SD between two lines. Then l + m - n = 0, l - m + 0. n = 0. Solving for l, m, n we get l = m = n . 1 1 2 Therefore, the direction cosines of the line of the SD are 1 , 1 , 2 . 6 6 6  \ SD = ( x1 − x2 )l + ( y1 − y2 )m + ( z1 − z2 )n

= ( 0 − 0) 1 + ( 0 − 0) 1 + ( a − 0) 2 = 2a 6 6 6 6

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Straight Line

13-45

The equation of the plane containing the edge given by (13.77) and the line x y z of the SD is 1 1 −1 = 0 . 1 1 2 (i.e.)  x + y – z + a = 0



Therefore, the equations of the SD are given by x – y = 0, x + y – z + a = 0



(13.79)

This line passes through the point (a, a, a). Similarly, by symmetry we note that the other two lines of SD also pass through the point (a, a, a).

Example 13.30 A square ABCD of diagonal 2a is folded along the diagonal AC, so that the planes DAC, BAC are at right angles. Find the shortest distance between DC and AB.

Solution Let a be the side of the square. Let us take OB, OC, OD as the axes of coordinates. The coordinates of B, C, D and A are (a, 0, 0), (0, a, 0), (0, 0, a), (0, 0, -a). z (0, 0, a) D

(0, 0, −a)

A

O

(0, a, 0) C

x

y

B (a, 0, 0)

The equations of AB are

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 45

x − a = y = z 1 1 0

(13.80)

4/22/2013 8:23:39 PM

13-46

Analytical Geometry

The equations of CD are

x = y−a = z 0 1 −1



(13.81)

The equations of the plane passing through the straight line (13.80) and x−a y z parallel to (13.81) is 1 1 0 = 0. 0 1 −1

-(x – a) + y + z = 0 or x – y – z –a = 0

(13.82)

Therefore, the required shortest distance = perpendicular from the point (0, a, 0) to the plane (13.82). = −2a = 2a units 3 3



Example 13.31 Prove that the shortest distance between the diagonal of rectangular parallelepiped and the edge not meeting it is b, c are the edges of the parallelepiped.

bc , ca , 2 ab 2 where a, b2 + c 2 a2 + b2 a + b

Solution Let OA, OB and OC be the coterminous edges of a rectangular parallelepiped. The diagonals are OO′, AA′, BB′ and CC′. The coordinates of O′ are (a, b, c). The coordinates of B and C′ are (a, 0, 0) and (a, b, 0). C′ (0, 0, c) B′ (0, a, b)

A′ O′ (a, b, c)

O

z y

x

A

B (a, 0, 0) C (a, b, 0)

y The equations of OO′ are x = = z . a b c The equations of BC are

M13_ANALYTICALGEOMETRY_ISBN_CH13.indd 46

( x − a) y z = = . b 0 0

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Straight Line

13-47

Let l, m, n be the direction cosines of the line of the SD. Then la + mb + nc = 0

0 + mb + 0 = 0 ⇒ m = 0 and l = −c n a Hence, l, m, n are –c, o, a.  The direction cosines of the line of the SD are  −c , 0,  c 2 + a2   ( −c)  Length of the SD = (0 − a) ⋅   + (0 − b)(0) + (0 − 0) ⋅  c 2 + a2    ac = c 2 + a2 bc Similarly we can prove that the other two SD are and b2 + c 2

 a 2  a +c   a  a 2 + c 2  2

ab . a2 + b2

Exercises 3 1.

2.

Prove that the lines x + 1 = y − 3 = z + 2 and x = y − 7 = z + 7 are coplanar −3 2 1 1 −3 2 and find the equation of the plane containing the line. Ans.: x - y + z = 0 Prove that the lines x + 1 = y + 3 = z + 5 and x − 2 = y − 4 = z = 6 3 5 7 2 3 5 intersect. Find the point of intersection and the plane containing the line. Ans.:

3.

4.

5.

( 12 , −21 , −23), x - 2y + z = 0

x − 1 y + 1 z + 10 = = intersect Show that the lines x − 4 = y + 3 = z + 1 and 1 −4 2 7 −3 3 and find the equation of the plane containing the lines. Ans.: (5, -7, 6) Prove that the lines x − 4 = y − 3 = z − 2 and x − 3 = y − 2 = z − 1 are 5 −2 −6 4 −3 −7 coplanar. Find also the point of intersection and the equation of the plane through them. Ans.: (-1, 5, 8), 4x – 11y + 7z + 3 = 0 Show that the lines x + 3 = y + 5 = z − 7 and x + 1 = y + 1 = z + 1 are 2 3 −3 4 5 −1 coplanar. Find the equation of the plane containing the line. Ans.: x - 2y + z = 0

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13-48

6.

Analytical Geometry

Show that the lines x − 5 = y − 7 = z + 3 and x − 8 = y − 4 = z − 5 are 4 4 −5 1 1 3 coplanar. Find the point of intersection and the equation of the plane containing them. Ans.: (1, 3, 2), 17x - 47y - 24z + 172 = 0

7.

Show that the lines x − 1 = y − 2 = z − 3 and x − 2 = y − 3 = z − 4 are 2 3 4 3 4 5 coplanar and find the equation of the plane containing them. Ans.: x - 2y + z = 0

8.

Show that the lines x + 1 = y + 2 = z − 4 and x − 1 = y − 2 = z + 4 are 2 3 −3 4 5 −1 coplanar and find the equation of the plane containing them. Ans.: 6x - 5y - z = 0

9.

Show that the lines x − 1 = y − 1 = z + 2 and x + 2 = y − 5 = z + 3 intersect 1 2 3 2 −1 3 and find the equation of the plane containing these lines. Ans.: (2, 3, 1), x − 2 = y − 3 = z − 1 7 4 −5

10.

Show that the lines x + 5 = y + 4 = z − 7 and 3x + 2y + z - 2 = 0, x - 3y + 3 1 −1 2z - 13 = 0 intersect. Find also the equation of the plane containing them. Ans.: (-1, 2, 3), 6x - 5y - z = 0.

11.

Show that the lines x -3y + 2z + 4 = 0, 2x + y + 4z + 1 = 0 and 3x + 2y + 5z - 1 = 0, 2y + z = 0 are coplanar. Find their point of intersection and the equation of the plane containing these lines. Ans.: (3, 1, -2), 3x + 4y + 6z -1 = 0.

12.

Show that the lines x + y + z - 3 = 0, 2x + 3y + 4z - 5 = 0 and 4x - y + 5z - 7 = 0, 2x - 5y - z - 3 = 0 are coplanar. Find the equation of the plane containing these lines. Ans.: x + 2y + 3z - 2 = 0.

13.

Show that the lines 7x - 4y + 7z + 16 = 0, 4x + 3y - 2z + 3 = 0 and x - 3y + 4z + 6 = 0, x - y + z + 1 = 0 are coplanar.

14.

Show that the lines 7x - 2y - 2z + 3 = 0, 9x - 6y + 3 = 0 and 5x - 4y + z = 0, 6y - 5z = 0 are coplanar. Find the equation of the plane in which they lie. Ans.: x - 2y + z = 0

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Straight Line

15.

13-49

Show that the lines x − 1 = y − 1 = z + 6 and x + 2y + z + 2 = 0, 4x + 5y + 2 −1 −3 3z + 6 = 0 are coplanar. Find the point of intersection of these two lines. Ans.: 5 , 5 , − 7 3 3 y + 1 x + 1 z + 1 Show that the lines and x + 2y + 3z - 14 = 0, 3x + 4y + = = 2 3 4 5z - 26 = 0 are coplanar. Find their point of intersection and the equation of the plane containing them.

(

16.

)

Ans.: (1, 2, 3), 11x + 2y - 7z + 6 = 0. 17.

Show that the lines 3x - y - z + 2 = 0, x - 2y + 3z - 6 = 0 and 3x - 4y + 3z 4 = 0, 2x - 2y + z - 1 = 0 are coplanar. Find their point of intersection and the equation of the plane containing these lines. Ans.: (1, 2, 3), x - z + 2 = 0

18.

Show that the lines 2x - y - z - 3 = 0, x - 3y + 2z - 4 = 0 and x - y + z - 2 = 0, 4x + y - 6z - 3 = 0 are coplanar and find the equation of the plane containing these two lines. Ans.: (1, -1, 0), x - z - 1 = 0

19.

Show that the lines x + 2y + 3z - 4 = 0; 2x + 3y + 4z - 5 = 0 and 2x - 3y + 3z - 5 = 0, 3x - 2y + 4z - 6 = 0 are coplanar. Find the equation of the plane containing these two lines. Ans.: x + y + z - 1 = 0

20.

Show that the equation of the plane through the line x = y = z and which is l m n perpendicular to the plane containing the lines x = y = z and x = y = z is m n l n l m (m - n) x + (n - l) y + (l - m) z = 0.

21.

Prove that the lines

22.

Show that the lines x = y = z , x = y = z and x = y = z are coplanar a b g aa bb cg l m n l m n if (b − c) + (c − a) + ( a − b) = 0 . a b g

23.

A, A′; B, B′ and C, C′ are points on the axes, show that the lines of intersection of the planes (A′BC, AB′C′), (B′CA, BC′A′) and (C′AB, CA′B′) are coplanar.

x − x1 y − y1 z − z1 and ax + by + cz + d = 0, a1x + = = n l m ax + by1 + cz1 + d a1 x1 + b1 y1 + c1 z1 + d1 b1y + c1z + d1 = 0, are coplanar if 1 . = al + bm + cz a1l + b1m + c1 z

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13-50

24.

25.

Analytical Geometry

Find the shortest distance between the lines x − 0 = y = z and −3 1 2 x − 2 = y − 1 = z + 2 and also the equations of the line of the SD. 3 −5 2 Ans.: 1 , 4x + y - 5z = 0, 9x + y - 8z - 31 = 0 3 Find the shortest distance between the lines x − 3 = y − 8 = z − 3 and −3 1 −1 x + 3 = y + 7 = z − 6 and find the equation of the line of the shortest 5 −2 −4 distance. Ans.: 3 3, 4x - 5y - 17z + 79 = 0, 22x - 5y + 19z - 83 = 0

26.

27.

Find the shortest distance between the lines x − 3 = y − 5 = z − 7 and 1 −2 1 x + 1 = y + 1 = z + 1 . Find also the equation of the line of SD and the points 7 −6 1 where the line of SD intersect the two given lines. Ans.: 2 29, (3, 5, 7), (-1, -1, -1) x − 3 = y − 5 = z − 7 2 3 4 Show that the shortest distance between z-axes and the line of intersection of the plane 2x + 3y + 4z - 1 = 0 with 3x + 2y + z - 2 = 0 is 7 5 . 25

28.

29.

Show that the shortest distance between the lines x − 1 = y − 2 = z − 3 and 2 3 4 x − 2 = y − 4 = z − 5 is 1 and its equation are 11x + 2y - 7z + 6 = 0, 7x + 6 3 4 5 y - 3z + 7 = 0. Find the length of the shortest distance between the lines x − 2 = y + 1 = z 2 3 4 and 2x + 3y - 6z - 6 = 0, 3x - 2y - z+ 5 = 0. Ans.: 97 3 78

30.

Find the shortest distance between z-axis and the line ax + by + cz + d = 0, a′x + b′y + c′z + d′ = 0. Ans.:

31.

dc′ − d ′c ( ac′ − a′c)2 + (bc′ − b′c)2

Find the shortest distance between an edge of a cube and a diagonal which does not meet it. Ans.: a 3

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Straight Line

32.

13-51

A line with direction cosines proportional to 1, 7, -5 is drawn to intersect the lines x − 5 = y − 7 = z + 2 and x + 3 = y − 3 = z − 6 . Find the coordinates 3 −1 1 −3 2 4 of the point of intersection and the length intercepted on it. Ans.: (2, 8, -3), (0, 1, 2),

33.

78

A line with direction cosines proportional to 2, 7, -5 is drawn to intersect the lines x − 5 = y − 7 = z + 2 , x + 3 = y − 3 = z . Find the coordinates of the 3 1 1 −3 2 4 points of intersection and the length intercepted on it. Ans.: (2, 8, -3), (0, 1, 2); 78

34.

The two lines x − a = y − b = z − g and x − a ′ = y − b ′ = z − g ′ are cut l m n l′ m′ n′ by a third line whose direction cosines are l, m, g. Show that the length a − a ′ b − b ′ g −′ g intercepted on the third line is given by l m n ÷ l′ m′ n′ l m n l ′ m′ n′ and show that the length of the shortest distance is d m n a −a ′ b − b ′ g −g ′ l m n ÷ l′ m′ n′

∑ (mn′ − m′n)

2

.

35.

The lengths of two opposite edges of a tetrahedron are a, b, c; the shortest

36.

distance is equal to d and the angle between them is q. Prove that the volume 1 of the tetrahedron is abd sinq. 6 Show that the equation of the plane containing the line x = 0, y + z = 1 and b c parallel to the line y = 0, x − z = 1 is x − y − z + 1 = 0. If d is the shortest a c a b c

37.

distance between the lines then show that 4 = 1 − 1 + 1 . d 2 a2 b2 c2 Show that the shortest distance between the lines y = az + b, z = a x + b and y (a − a ′ )(b − b′ ) − (a ′b − ab ′ )( a − a′ ) . = a′z + b, z = a ′x + b ′y is 2 [a − a ′ 2 ( a − a′ )2 + (a − a 2 ) + ( aa − a′a ′ )2 ]1/ 2

38.

Find the shortest distance between the lines x = 2z + 3, y = 3z + 4 and x = 4z + 5, y = 5z + 6. What conclusion do you draw from your answer? Ans.: Zero; Coplanar lines

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Chapter

14

Sphere

14.1  Definition of Sphere The locus of a moving point in space such that its distance from a fixed point is constant is called a sphere. The fixed point is called the centre of the sphere. The constant distance is called the radius.

14.2  The equation of a sphere with centre at   (a, b, c ) and radius r Let P(x, y, z) be any point on the sphere. Let C(a, b, c) be the centre. P(x, y, z) r C (a, b, c)

Then,

CP = r



CP2 = r2



(x - a)2 + (y - b)2 + (z - c)2 = r2

This is the equation of the required sphere. Show that the equation x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 always represents a sphere. Find its centre and radius.

x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 x2 + y2 + z2 + 2ux + 2vy + 2wz = -d

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14-2

Analytical Geometry

Add u2 + v2 + w2 to both sides. x2 + y2 + z2 + 2ux + 2vy + 2wz + u2 + v2 + w2 = u2 + v2 + w2 - d (x2 + 2ux + u2) + (y2 + 2vy + v2) + (z2 + 2wz + w2) = u2 + v2 + w2 - d

(i.e.)  (x + u)2 + (y + v)2 + (z + w)2 =

(

u2 + v2 + w 2 − d

)

2

This equation shows that this is the locus of a point (x, y, z) moving from the fixed point (-u, -v, -w) keeping a constant distance u 2 + v 2 + w 2 − d from it. Therefore, the locus is a sphere whose centre is (-u, -v, -w) and whose radius is u 2 + v 2 + w 2 − d .

Note 14.2.1: A general equation of second degree in x, y, z will represent a sphere if (i) coefficients of x2, y2, z2 are the same and (ii) the coefficients of xy, yz, zx are zero.

14.3  Equation of the sphere on the line joining the points (x 1, y 1, z 1) and (x 2, y 2, z 2) as diameter Find the equation of the sphere on the line joining the points (x1, y1, z1) and (x2, y2, z2) as the extremities of a diameter. P(x, y, z)

A (x1, y1, z1)

B (x2, y2, z2)

A (x1, y1, z1) and B (x2, y2, z2) be the ends of a diameter. Let (x, y, z) be any point on the surface of the sphere. Then ∠APB = 90° Therefore, AP is perpendicular to BP. The direction ratios of AP are x - x1, y - y1, z - z1. The direction ratios of BP are x - x2, y - y2, z - z2. Since AP is perpendicular to BP, (x - x1)(x - x2) + (y - y1)(y - y2) + (z - z1)(z - z2) = 0 This is the equation of the required sphere.

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Sphere

14-3

14.4  Length of the tangent from P(x1, y1, z1) to the sphere x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d = 0 Find the length of the tangent from P(x1, y1, z1) to the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. The centre of the sphere is (-u, -v, -w). u2 + v2 + w 2 − d .

The radius of the sphere is

P(x1, y1, z1)

T

C (−u, −v, −w)



PC 2 = (x1 + u)2 + (y1 + v)2 + (z1 + w)2 ∠CTP = 90° PT 2 = PC 2 - CT 2 = (x1 + u)2 + (y1 + v)2 + (z1 + w)2 - (u2 + v2 + w2 - d) = x12 + y12 + z12 + 2ux1 + 2vy1 + 2wz1 + d ∴ PT =

x12 + y12 + z12 + 2ux1 + 2vy1 + 2wz1 + d

Note 14.4.1: If PT 2 > 0, the point P lies outside the sphere.

If PT 2 = 0, then the point P lies on the sphere. If PT 2 < 0, then the point P lies inside the sphere.

14.5  Equation of the tangent plane at (x1, y1, z1) to the sphere x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d = 0 Find the equation of the tangent plane at (x1, y1, z1) to the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. The centre of the sphere is (-u, -v, -w). P(x1, y1, z1) is a point on the sphere and the required plane is a tangent plane to the sphere at P. Therefore, the direction ratios of CP are x1 + u, y1 + v, z1 + w.

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14-4

Analytical Geometry

(−u, −v, −w) C

P (x1, y1, z1)

Therefore, the equation of the tangent plane at (x1, y1, z1) is (x1 + u)(x - x1) + ( y1 + v)(y - y1) + (z1 + w)(z - z1) = 0. (i.e.)  xx1 + ux − ux1 − x12 + yy1 + vy − vy1 − y12 + zz1 + wz − wz1 − z12 = 0 (i.e.)  xx1 + yy1 + zz1 + ux + vy + wz = x12 + y12 + z12 + ux1 + vy1 + wz1 Adding ux1 + vy1 + wz1 + d to both sides, we get xx1 + yy1 + zz1 + u(x + x1) + v(y + y1) + w(z + z1) + d = x 2 + y 2 + z 2 + 2ux + 2vy + 2wz + d 1



1

1

1

1

1

= 0 since (x1, y1, z1) lies on the sphere.

Therefore, the equation of the tangent plane at (x1, y1, z1) is xx1 + yy1 + zz1 + u(x + x1) + v(y + y1) + w(z + z1) + d = 0.

14.6  Section of a sphere by a plane Let C be the centre of the sphere and P be any point on the section of the sphere by the plane. Draw CN perpendicular to the plane. Then N is the foot of the perpendicular from P on the plane section. Join CP. Since CN is perpendicular to NP, CNP is a right angled triangle.

CP2 = CN2 + NP2 or



NP2 = CP2 - CN2



NP =

CP 2 − CN 2 = a constant N

P

C

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Sphere

14-5

Since CP and CN are constants, NP = constant shows that the locus of P is a circle with centre at N and radius equal to NP.

Note 14.6.1: If the radius of the circle is less than the radius of the sphere then the circle is called a small circle. In other words, a circle of the sphere not passing through the centre of the sphere is called a small circle.

Note 14.6.2: If the radius of the circle is equal to the radius of the sphere then the circle is called a great circle of the sphere. In other words, a circle of the sphere passing through the centre of the sphere is called a great circle.

14.7  Equation of a circle The section of a sphere by a plane is a circle. Suppose the equation of the sphere is

x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0

(14.1)

and the plane section is ax + by + cz + k = 0



(14.2)

Then any point on the circle lie on the sphere (14.1) as well as the plane section (14.2). Hence, the equations of the circle of the sphere are given by x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 and ax + by + cz + k = 0.

14.8  Intersection of two spheres The curve of intersection of two spheres is a circle. Let the two spheres be

S = x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0



S1 = x2 + y2 + z2 + 2u1x+ 2v1y+ 2w1z + d = 0



S - S1 = 2(u - u1)x + 2(v - v1)y + 2(w - w1)z + d - d1 = 0

(14.3)

Equation (14.3) is a linear equation in x, y, z and therefore represents a plane and this plane passes through the point of intersection of the given two spheres. In addition, we know that section of the sphere by a plane is a circle. Hence the curve of intersection of the spheres is given by S1 - S2 = 0.

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14-6

Analytical Geometry

14.9  Equation of a sphere passing through a given circle Let the given circle be S = x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 and P = ax + by + cz + k = 0



(14.4)

Consider the equation S + l P = 0. (i.e.)  x2 + y2 + z2 + 2ux + 2vy + 2wz + d + l(ax + by + cz + k) = 0

(14.5)

This equation represents a sphere. Suppose (x1, y1, z1) is a point on the given circle. Then

x12 + y12 + z12 + 2ux1 + 2vy1 + 2wz1 + d = 0 ax1 + by1 + cz1 + k = 0



(14.6) (14.7)

Equations (14.6) and (14.7) show that the point (x1, y1, z1) lies on the sphere given by equation (14.5). Since (x1, y1, z1) is an arbitrary point on the circle, it follows that every point on the circle is a point on the sphere given by (14.5). Hence equation (14.5) represents the equation of a sphere passing through the circle (14.4).

14.10  Condition for orthogonality of two spheres Let the two given spheres be

x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0

(14.8)



x2 + y2 + z2 + 2u1x + 2v1y + 2w1z + d1 = 0

(14.9)

The centres of the spheres are A(- u, - v, - w) and B(- u1, - v1, - w1). The radius r = u2 + v2 + w 2 − d .

r1 =

u12 + v12 + w12 − d1 P r A

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r1 B

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14-7

Sphere

Two spheres are said to be orthogonal, if the tangent planes at this point of intersection are at right angles. (i.e.)  The radii drawn through the point of intersection are at right angles. ∴ AB2 = AP2 + BP2



(i.e.)  (-u + u1)2 + (-v + v1)2 + (-w + w1)2 = (u2 + v2 + w2 - d) + ( u12 + v12 + w12 − d1 )

(i.e.)  2uu1 + 2vv1 + 2ww1 = d + d1

This is the required condition.

14.11  Radical plane The locus of a point whose powers with respect to two spheres are equal is called the radical plane of the two spheres.

14.11.1  Obtain the Equations to the Radical Plane of Two Given Spheres Let the two given spheres be

x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0

(14.10)



x2 + y2 + z2 + 2u1x + 2v1y + 2w1z + d1 = 0

(14.11)

Let (x1, y1, z1) be a point such that the power of this point with respect to spheres (14.10) and (14.11) be equal. Then

x12 + y12 + z12 + 2ux1 + 2vy1 + 2wz1 + d



= x12 + y12 + z12 + 2u1 x1 + 2v1 y1 + 2w1 z1 + d1



(i.e.)  2(u - u1)x1 + 2(v - v1)y1 + 2(w - w1)z1 + (d - d1) = 0

The locus of (x1, y1, z1) is

2(u - u1)x + 2(v - v1)y + 2(w - w1)z + (d - d1) = 0

(14.12)

This is a linear equation in x, y and z and hence this equation represents a plane. Hence equation (14.12) is the equation to the radical plane of the two given spheres.

Note 14.11.1.1: When two spheres intersect, the plane of their intersection is the radical plane.

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14-8

Analytical Geometry

Note 14.11.1.2: When the two spheres touch, the common tangent plane through the point of contact is the radical plane.

14.11.2  Properties of Radical Plane 1. The radical plane of two spheres is perpendicular to the line joining their centres.

Proof: Let the equations of the two spheres be

x2 + y2 + z2 + 2u1x + 2v1y + 2w1z + d1 = 0

(14.13)



x2 + y2 + z2 + 2u2x + 2v2y + 2w2z + d2 = 0

(14.14)

The centres of the two spheres are C1(- u1, - v1, - w1) and C2(- u2, - v2, - w2).



The direction ratios of the line of centres are u1 - u2, v1 - v2, w1 - w2. The radical plane of spheres (14.13) and (14.14) is 2(u1 - u2)x + 2(v1 - v2)y + 2(w1 - w2)z + (d1 - d2) = 0. The direction ratios of the normal to the plane are u1 - u2, v1 - v2, w1 - w2. Therefore, the line of centre is parallel to the normal to the radical plane. Hence, the radical plane of two spheres is perpendicular to the line joining the centres. 2. The radical planes of three spheres taken in pairs pass through a line.

Proof: Let S1 = 0, S2 = 0, S3 = 0 be the equations of the three given spheres in each of which the coefficients of x2, y2 and z2 are unity. Then the equations of the radical planes taken in pairs are S1 - S2 = 0, S2 - S3 = 0, S3 - S1 = 0. These equation show that the radical planes of the three spheres pass through the line S1 = S2 = S3. Hence the result is proved.

Note 14.11.2.1: The line of concurrence of the three radical planes is called radical line of the three spheres. 3. The radical planes of four spheres taken in pairs meet in a point.

Proof:

Let S1 = 0, S2 = 0, S3 = 0 and S 4 = 0 be the equations of the four given spheres, in each of which the coefficients of x2, y2, z2 are unity.

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Sphere



14-9

Then the equations of the radical planes taken two by two are S1 - S2 = 0, S1 - S3 = 0, S1 - S4 = 0 S2 - S3 = 0, S2 - S4 = 0 and S3 - S4 = 0



These equations show that the radical planes of the four spheres meet in at a point given by S1 = S2 = S3 = S4.

Note 14.11.2.2: The point of concurrence of the radical planes of four spheres is called the radical centre of the four spheres.

14.12  Coaxal System Definition 14.12.1: A system of spheres is said to be coaxal if every pair of spheres of the system has the same radical plane.

14.12.1  General Equation to a System of Coaxal Spheres Let S = x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 and S1 = x2 + y2 + z2 + 2u′x + 2v′y + 2w′z + d′ = 0 be the equation of any two spheres. Now consider the equation S + l S′ = 0



(14.15)

where l is a constant. Clearly this equation represents a sphere. Consider two different spheres of this system for two different values of l.

(i.e.)  S + l1S′ = 0

(14.16)

S + l2S′ = 0

(14.17)

and

The coefficients of x2, y2, z2 terms in (14.15) are 1 + l. ∴

S + l 1S ′ =0 1+ l 1

   

S + l 2S ′ =0 1+ l 2



(14.18) (14.19)

represent two spheres of the system with unit coefficients for x2, y2, z2 terms. Therefore, the equation of the radical plane of (14.18) and (14.19) is S + l 1S ' S + l 2 S ' − = 0. 1+ l 1 1+ l 2

(1 + l2)(S + l1S′) - (1 + l1)(S + l2S′) = 0



(i.e.)  (l2 - l1)(S - S′) = 0

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14-10

Analytical Geometry

Since l2 ≠ l1, S - S′ = 0 which is the equation to the radical plane of spheres (14.16) and (14.17). Since this equation is independent of l, every pair of the system of spheres (14.15) has the same radical plane. Hence equation (14.15) represents the general equation to the coaxal system of the spheres.

14.12.2  Equation to Coaxal System is the Simplest Form In a coaxal system of spheres, the line of centres is normal to the common radical plane. Therefore, let us choose the x-axis as the line of centres and the common radical plane as the yz-plane, that is, (x = 0). Let the equation to a sphere of the coaxal system be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. Since the line of centres is the x-axis, is y and z coordinates are zero v = 0, w = 0. Then the equation of the above sphere reduces to the form x2 + y2 + z2 + 2ux + d = 0. Let us now consider two sphere of this system say, x2 + y2 + z2 + 2ux + d = 0 and x2 + y2 + z2 + 2u1x + d1 = 0. The radical plane of these two spheres is 2(u - u1)x + d - d1 = 0



(14.20)

But the equation of the radical plane is x = 0. Therefore, from (14.20), d - d1 = 0 or d1 = d Hence the equation to any sphere of the coaxal system is of the form x 2 + y 2 + z 2 + 2l x + d = 0 where l is a variable and d is a constant.

14.12.3  Limiting Points Limiting points are defined to be the centres of point spheres of the coaxal system. Let the equation to a coaxal system be

x 2 + y 2 + z 2 + 2l x + d = 0

Centre is (-l, 0, 0) and radius is

(14.21)

l2 − d.

For point sphere radius is zero.

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∴ l2 − d = 0 ∴l = ± d

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Sphere

14-11

Therefore, the limiting points of the system of spheres given by (14.21) are ( d , 0, 0) and ( − d , 0, 0).

Note 14.12.3.1: Limiting points are real or imaginary according as d is positive or negative. 14.12.4  Intersection of Spheres of a Coaxal System Let the equation to a coaxal system of sphere be x 2 + y 2 + z 2 + 2l x + d = 0 . Now consider two spheres of the system say Now consider two spheres of the system say

S1 = x 2 + y 2 + z 2 + 2l 1 x + d = 0

(14.22)



S 2 = x + y + z + 2l 2 x + d = 0

(14.23)

2

2

2

The intersection of these two spheres is S1 - S2 = 0.

(i.e.)  2( l 1 − l 2 ) x = 0

(i.e.)  x = 0 since l 1≠ l 2substituting x = 0 in (14.22) or (14.23) we get,

y2 + z 2 = −d



y2 + z2 =

(

−d

)

2

Therefore, this equation is a circle in the yz-plane and also it is independent of l. Hence every sphere of the system meets the radical plane with same circle.

Note 14.12.4.1: This circle is called the common circle of the coaxal system. Note 14.12.4.2:  If d < 0, the common circle is real and the system of spheres are said to be intersecting type. If d = 0, the common circle is a point circle and in this case any two spheres of the system touch each other. If d > 0, the common circle is imaginary and the spheres are said to be of non-intersecting type.

Illustrative Examples Example 14.1 Find the equation of the sphere with centre at (2, -3, -4) and radius 5 units.

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14-12

Analytical Geometry

Solution The equation of the sphere whose centre is (a, b, c) and radius r is (x - a)2 + (y - b)2 + (z - c)2 = r2. Therefore, the equation of the sphere whose centre is (2, -3, -4) and radius 5 is ( x − 2)2 + ( y + 3)2 + ( z − 4)2 = 52.

(i.e.)  x 2 + y 2 + z 2 − 4 x + 6 y − 8 z + 4 + 9 + 16 − 25 = 0



(i.e.)  x 2 + y 2 + z 2 − 4 x + 6 y − 8 z + 4 = 0

Example 14.2 Find the coordinate of the centre and 16 x 2 + 16 y 2 + 16 z 2 − 16 x − 8 y − 16 z − 35 = 0 .

radius

of

the

sphere

Solution The equation of the sphere is 16 x 2 + 16 y 2 + 16 z 2 − 16 x − 8 y − 16 z − 35 = 0 . 1 35 =0 Dividing by 16, x 2 + y 2 + z 2 − x − y − z − 2 16  1 1 1 Centre of the sphere is  , ,  .  2 4 2 Radius is r =

1 1 1 35 + + + . 4 16 4 16

=

4 + 1 + 4 + 35 44 = 16 16

=

11 11 = units 4 2



Example 14.3 Find the equation of the sphere with the centre at (1, 1, 2) and touching the plane 2x - 2y + z = 5.

Solution The radius of the sphere is equal to the perpendicular distance from the centre (1, 1, 2) on the plane 2x - 2y + z - 5 = 0.

r=

M14_ANALYTICALGEOMETRY_ISBN_CH14.indd 12

2−2+2−5 4 + 4 +1

=

3 = 1 unit 3

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Sphere

14-13

The equation of the sphere with centre at (1, 1, 2) and radius 1 unit is (x - 1)2 + (y - 1)2 + (z - 2)2 = 1. (i.e.)  x2 + y2 + z2 - 2x - 2y - 4z + 5 = 0

Example 14.4 Find the equation of the sphere passing through the points (1, 0, 0), (0, 1, 0), (0, 0, 1) and (0, 0, 0).

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This passes through the points (1, 0, 0), (0, 1, 0), (0, 0, 1) and (0, 0, 0). ∴ 2u + d + 1 = 0 2v + d + 1 = 0 2w + d + 1 = 0 d=0 \ 2u = -1, 2v = -1 and 2w = -7, d = 0 The equation of the sphere is x2 + y2 + z2 - x - y - z = 0.

Example 14.5 Find the equation of the sphere which passes through the points (1, 0, 0), (0, 1, 0) and (0, 0, 1) and has its centre on the plane x + y + z = 6.

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This sphere passes through the points (1, 0, 0), (0, 1, 0) and (0, 0, 1).

∴ 2u + d = 0 2v + d = 0 2w + d = 0

The centre of the sphere is (-u, -v, -w). This lies on the plane x + y + z - 6 = 0. \ - u - v - w = 6 d d d 3d = 6 or d = 4 ∴ + + = 6 or 2 2 2 2 \ u = v = w = -2 The equation of the sphere is x2 + y2 + z2 - 4x - 4y - 4z + 4 = 0.

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14-14

Analytical Geometry

Example 14.6 Find the equation of the sphere touching the plane 2x + 2y - z = 1 and concentric with the sphere 2x2 + 2y2 + 2z2 + x + 2y - z = 0.

Solution 2x2 + 2y2 + 2z2 + x + 2y - z = 0 or x 2 + y 2 + z 2 +

1 1 x+ y− z = 0 2 2

 −1 −1 1  Centre is  , ,  .  4 2 4 The sphere touches the plane 2x + 2y - z -1 = 0. ∴r =



−2 2 1 − − −1 4 2 4 4 + 4 +1 2

=

11 units. 12 2

2

1  1  1   11  The equation of the sphere is  x +  +  y +  +  z −  =     12  4  2  4

2

Example 14.7 Find the equation of the sphere which passes through the points (2, 7, -4) and (4, 5, -1) has its centre on the line joining the these two points as diameter.

Solution

 2 + 4 7 + 5 −4 − 1 , , Centre of the sphere =    2 2 2 

−5   =  3, 6,   2

1 1 17 4+4+9 = (2 − 4)2 + (7 − 5)2 + ( −4 + 1)2 = . 2 2 2 2 5 17  2 2 ( x − 3 ) + ( y − 6 ) + z + Therefore, the equation of the sphere is   = . 2 4 Radius =



25 17 − =0 4 4 x 2 + y2 + z2 - 6x - 12y + 5z + 47 = 0

x2 + y2 + z2 - 6x - 12y + 5z + 9 + 36 +

Aliter: The two given points are the extremities of a diameter of the sphere.

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Sphere

14-15

Therefore, the equation of the sphere is

(x - 2)(x - 4) + ( y - 7)( y - 5) + (z + 4)(z + 1) = 0 (i.e.)  x2 + y2 + z2 - 6x - 12y + 5z + 47 = 0

Example 14.8

x y z + + = 1 cuts the coordinate axes in A, B and C. Find the a b c equation of the sphere passing through A, B, C and O. Find also its centre and radius.

The plane

Solution

x y z + + = 1 cuts the coordinates of A, B and C. The coordinates a b c of A, B and C are (a, 0, 0), (0, b, 0) and (0, 0, c).

The plane

z C

y B x

A

Let the equation of the sphere passing through A, B and C be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. Since this passes through origin O, d = 0. Since this passes through A, B and C.

a2 + 2ua = 0, b2 + 2vb = 0, c2 + 2wc = 0. ∴  2u = -a, 2v = -b and 2w = -c.

Hence the equation of the sphere is x2 + y2 + z2 - 2ax - 2by - 2cz = 0. Centre of the sphere is (a, b, c) and radius of the sphere = a 2 + b 2 + c 2 .

Example 14.9 Find the equation of the sphere circumscribing the tetrahedron whose faces y z z x x y x y z are + = 0, + = 0, + = 0 and + + = 1. b c c a a b a b c

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14-16

Analytical Geometry

Solution The faces of the tetrahedron are

y z + = 0 b c

(14.24)



z x + = 0 c a

(14.25)



x y + = 0 a b

(14.26)



x y z + + − 1 = 0 a b c

(14.27)

Now easily seen that the vertices of the tetrahedron are (0, 0, 0), (a, b, -c), (a, -b, c) and (-a, b, c). Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This sphere passes through the points (0, 0, 0), (a, b,-c,). (a, -b, c) and (-a, b,c).

∴d=0



a2 + b2 + c2 + 2ua + 2bv - 2cw = 0

(14.28)



a + b + c + 2ua - 2bv + 2cw = 0

(14.29)



a2 + b2 + c2 - 2ua + 2bv + 2cw = 0

(14.30)

2

2

2

Adding (14.28) and (14.29), 2(a2 + b2 + c2) + 4ua = 0

∴ 2u =

−( a 2 + b 2 + c 2 ) a

−( a 2 + b 2 + c 2 ) −( a 2 + b 2 + c 2 ) , 2w = c b Therefore, the equation of the sphere is x2 + y2 + z2 - (a2 + b2 + c2) Similarly, 2v =

 x y z  + +  = 0. a b c

Example 14.10 A sphere is inscribed in a tetrahedron whose faces are x = 0, y = 0, z = 0 and 2x + 6y + 3z = 14. Find the equation of the sphere. Also find its centre and radius.

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Sphere

14-17

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. Since the sphere touches the plane x = 0, the perpendicular distance from the centre (-u, -v, -w) on this plane is equal to the radius. \  -u = r, -v = r, -w = r.



Also the sphere touches the plane 2x + 6y + 3z - 14 = 0. ∴

−2u − 6v − 3w − 14 4 + 36 + 9 (i.e.)

=r

11r − 14

=r 7 ∴ 7r = ±(11r − 14) 7r = 11r - 14  or  7r = -11r + 14



∴ 4 r = 14 or r =



r=



14 7 = 18 9

7 7 ;r= 9 2 2

When r = 2

2

7 7 7 , the equation of the sphere is  x −  +  y −  + 9  9  9 2

7   7  z −  =   . 9 9

(i.e.) 

x2 + y2 + z2 −

14 x + 14 y + 14 z 49 +2× =0 9 81

81(x2 + y2 + z2) - 126 (x + y + z) + 98 = 0

7  7 7 7 For this sphere, centre is  , ,  and radius = .  9 9 9 9 7 When r = , 2 2



2

2

7  7  7  7   x −  +  y −  +  z −  =   2 2 2 2 49 =0 x 2 + y 2 + z 2 − 7( x + y + z ) + 2 (i.e.) 2( x 2 + y 2 + z 2 ) − 14( x + y + z ) + 49 = 0

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2

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14-18

Analytical Geometry

Example 14.11 Find the equation of the sphere passing through the points (1, 0, -1), (2, 1, 0), (1, 1, -1) and (1, 1, 1).

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This sphere passes through (1, 0, -1), (2, 1, 0), (1, 1, -1) and (1, 1, 1).

\  1 + 0 + 1 + 2u + 0 - 2w + d = 0



4 + 1 + 0 + 4u + 2v + 0 + d = 0



1 + 1 + 1 + 2u + 2v - 2w + d = 0



1 + 1 + 1 + 2u + 2v + 2w + d = 0



2u - 2w + d = -2

(14.31)



4u + 2v + d = -5

(14.32)



2u + 2v - 2w + d = -3

(14.33)



2u + 2v + 2w + d = -3

(14.34)

(14.33) - (14.34) gives  -4w = 0  \  w = 0 (14.32) - (14.33) gives  2u + 2w = -2 \ 2u = -2 ⇒ u = -1 (14.32) - (14.31) gives  2u + 2v + 2w = -3 ⇒ -2 + 2v + 0 = -3

2v = −1 ⇒ v =

−1 2

From (14.31), -2 + d = -2 ⇒ d = 0. Therefore, the required equation of the circle is x2 + y2 + z2 - 2x - y = 0.

Example 14.12 Find the equation of the sphere which touches the coordinate axes, whose centre lies in the positive octant and has a radius 4.

Solution Let the equation of the sphere be x2 + y2 + z2 + 2xu + 2vy + 2wz + d = 0. x y z The equation of the x- axis is = = = t . 1 0 0

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Sphere

14-19

Any point on this line is (t, 0, 0). The point lies on the given sphere t2 + 2ut + d = 0. Since the sphere touches the x-axis the two roots of this equation are equal. \ 4u2 - 4d = 0  or  u2 = d.



Similarly, v2 = d and w2 = d The radius of the sphere is u 2 + v 2 + w 2 − d = 3d − d = 2d . 2d = 4 or d = 8  u2 = v2 = w2 = 8.



Since the centre lies on the x-axis, -u = -v = -w = 2 2. Therefore, the required equation is x 2 + y 2 + z 2 − 4 2 ( x + y + z ) + 8 = 0 .

Example 14.13 Find the radius and the equation of the sphere touching the plane 2x + 2y z = 0 and concentric with the sphere 2x2 + 2y2 + 2z2 + x + 2y - z = 0.

Solution Since the required sphere is concentric with the sphere 2x2 + 2y2 + 2z2 + x + 2y -z = 0 its centre is the same as that of the given sphere 1 1 x 2 + y 2 + z 2 + x + y − z = 0. 2 2  −1 −1 1  Centre is  , ,  . The radius of the required sphere is equal to the per 4 2 4 pendicular distance from this point to the plane 2x + 2y - z = 0.

(i.e.) 



r=

 −1 1  −1 2  + 2  −  2 4  4 4 + 4 +1

=

7 12 2

The

equation 2

of

the

2

1   7  z −  =   . 4 12

required

sphere

is

2

1  1   x +  +  y +  + 4 2

Example 14.14 Find the equation of the sphere which passes through the points (1, 0, 0), (0, 2, 0), (0, 0, 3) and has its radius as small as possible.

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14-20

Analytical Geometry

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + k = 0. This sphere passes through the points (1, 0, 0), (0, 2, 0) and (0, 0, 3).  k + 1 ∴ 2u + k + 1 = 0 ∴ u = −   2   k + 4 4v + k + 4 = 0 ∴ v = −   4 



 k + 9 6w + k + 9 = 0 ∴ w = −   6  The radius of the sphere is given by r2 = u2 + v2 + w2 - k. r2 =

( k + 1)2 ( k + 4)2 ( k + 9)2 36( k + 1)2 + 9( k + 4)2 + 4( k + 9)2 − 144 k + + −k = 4 16 36 144

=

1 49  2 72 504  [49k 2 + 72k + 504] = k + k+ 144 144  49 49 

=

2 2 49  36  504  36   k + + −      144  49  49  49  

r is minimum if k =

−36 49

 −36   −36   −36   49 + 1 13  49 + 4  40  49 + 9  135 ;w = −  ∴u = −   = ;v = −  = =  2  98  4  49  6  98       The required equation 80 135 36 y− z− = 0. 49 49 49

of

the

sphere

is

x2 + y2 + z2 −

13 x− 49

(i.e.)  49( x 2 + y 2 + z 2 ) − 13 x − 80 y − 135 z − 36 = 0

Example 14.15 Find the equation of the sphere tangential to the plane x - 2y - 2z = 7 at (3, -1, -1) and passing through the point (1, 1, -3).

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Sphere

Solution The equation of normal at A is

14-21

x − 3 y +1 z +1 = = = r. 1 −2 −2

C

B(1, 1, −3)

A (3, −1, −1) x − 2y − 2z = 7

Any point in this line is (r + 3, -2r -1, -2r - 1). If this point is the centre of the sphere then CA = CB.

r 2 + 4 r 2 + 4 r 2 = ( r + 2)2 + ( −2r − 2)2 + ( −2r + 2)2 ∴ 4 r = −12 or r = −3.

Therefore, centre of the sphere is (0.5, 5). Radius = 9r 2 = 81 = 9 Therefore, the equation of the sphere is (x - 0)2 + (y - 5)2 + (z - 5)2 = 81.

(i.e.)  x2 + y2 + z2 - 10y - 10z - 31 = 0

Example 14.16 Show that the plane 4x - 3y + 6z - 35 = 0 is a tangent plane to the sphere x2 + y2 + z2 - y - 2z - 14 = 0 and find the point of contact.

Solution If the plane is a tangent plane to the sphere then the radius is equal to the perpendicular distance from the centre on the plane.

(0, 1 ,1) 2

4x − 3y + 6z − 35 = 0

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14-22

Analytical Geometry

 1  The centre of the sphere x2 + y2 + z2 -y - 2z - 14 = 0 is  0, ,1 .  2  1 61 r = 0 + + 1 + 14 = 4 4 Perpendicular distance from the centre on the plane is  1 4(0) − 3   + 1(6) − 35  2



16 + 9 + 36

=

61 . 2

Therefore, the plane touches the sphere. The equations of the normal to the tangent plane are

x y − 12 z − 1 = = = t. 4 6 −3

1   Any point on this line is  4t , −3t + , 6t + 1 .   2 If this point lies on the plane 4x - 3y + 6z - 35 = 0 then,

1  4( 4t ) − 3  −3t +  + 6(6t + 1) − 35 = 0 .  2



3 61 1 16t + 9t + 36t − + 6 − 35 = 0 (i.e.) 61t − = 0, t = 2 2 2

Therefore, the point of contact is (2, -1, 4).

Example 14.17 A sphere of constant radius r passes through the origin O and cuts the axes in A, B and C. Find the locus of the foot of the perpendicular from O to the plane ABC.

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This passes through the origin. ∴  d = 0 The sphere cuts the axes at A, B and C where it meets the x-axis. y = 0, z = 0  ∴  x2 + 2ux = 0  ∴  x = -2u Therefore, the coordinates of A are (-2u, 0, 0). Similarly the coordinates of B and C are B(0, -2v, 0) and C(0, 0, -2w).

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Sphere

14-23

Therefore, the equations of the sphere is x2 + y2 + z2 - 2ux - 2vy - 2wz = 0. Radius = r \  u2 + v2 + w2 = r2



(14.35)

The equations of the plane ABC is x y z + + = 1 −2u −2v −2w



(14.36)

 −1 −1 −1  The direction ratios of the normal to this plane are  , ,  .  2u 2v 2w  The equations of the normal are

-2ux = -2vy = -2wz = t

(14.37)

Let (x, y, z) be the foot of the perpendicular from O on the plane. Then (x1, y1, z1) lies on (14.37). ∴ ux1 = vy1 = wz1 =



Substituting in (14.35),

−t −t −t −t or u = ,v = ,w = 2 2 x1 2 y1 2 z1

t 2 −2 ( x1 + y1−2 + z1−2 ) = r 2 4 t 2 ( x1−2 + y1−2 + z1−2 ) = 4 r 2



(14.38)

The point (x1, y1, z1) also lies on the plane (14.36)

∴

x1 y z + 1 + 1 =1 −2u −2v −2w

⇒

x12 y12 z12 + + =1 t t t

or 1 2 1 ( x1 + y12 + z12 ) = 1 or 2 ( x12 + y12 + z12 )2 = 1 t t Multiplying (14.38) and (14.39), we get





(14.39)

( x12 + y12 + z12 )2 ( x1−2 + y1−2 + z1−2 ) = 4 r 2

The locus of (x1, y1, z1) is (x2 + y2 + z2)2 (x-2 + y-2 + z-2) = 4r2.

Example 14.18 A sphere of constant radius 2k passes through the origin and meets the axes in A, B and C. Show that the locus of the centroid of the tetrahedron OABC is x2 + y2 + z2 = k2.

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14-24

Analytical Geometry

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This passes through the origin. \d=0



When this sphere meets the x-axis, y = 0, z = 0. \  x2 + 2ux = 0. Since x ≠ 0, x = -2u. Therefore, the coordinates of A are (-2u, 0, 0). Similarly the coordinates of B and C are (0, -2v, 0) and (0, 0, -2w). Let (x1, y1, z1) be the centroid of the tetrahedron OABC. −2u , u = −2 x1 4 −2v , v = −2 y1 y1 = 4 −2w , w = −2 z1 z1 = 4 x1 =

(14.40)

The radius of the sphere is r. \  u2 + v2 + w2 = 4r2 Using (14.40), x12 + y12 + z12 = r 2 The locus of (x1, y1, z1) is the sphere x2 + y2 + z2 = r2.

Example 14.19 A sphere of constant radius r passes through the origin and meets the axes in A, B and C. Prove that the centroid of the triangle ABC lies on the sphere 9(x2 + y2 + z2) = 4r2.

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. This line passing through the origin.

∴ d = 0.

When the circle meets the x-axis, y = 0, z = 0

M14_ANALYTICALGEOMETRY_ISBN_CH14.indd 24

∴ x 2 + 2ux = 0

4/22/2013 8:19:32 PM

14-25

Sphere

x ≠ 0, x = −2u

as

\ A is the point (-2u, 0, 0). Similarly B and C are the points (0, -2v, 0) and (0, 0, -2w). Also given the radius is r. \ u2 + v2 + w2 = r2

(14.41)

Let (x1,y1,z1) be the centroid of the triangle ABC.  −2u −2v −2w  , , But the centroid is  .  3 3 3 

−3 x1 −2u = x1  ∴ u = 3 2 −3 y1 −2v = y1   ∴ v = ∴ 2 3 −3 z1 −2w . = z1   ∴ w = ∴ 2 3 ∴

9 2 ( x1 + y12 + z12 ) = r 2 . 4 The locus of (x1, y1, z1) is 9(x2 + y2 + z2) = 4r2.

∴ from (14.41):

Example 14.20 A plane passes through the fixed point (a, b, c) and meets the axes in A, B, C. a b c Prove that the locus of the centre of the sphere is + + = 2. x y z

Solution Let the equation of the sphere be x2 + y2 + z2 + 2ux+ 2vy + 2wz + d = 0. This passes through the origin. ∴ d=0



When this sphere meets the x-axis, y = 0 and z = 0.

∴  x2 + 2ux = 0. As x ≠0, x = -2u.

Therefore, the coordinates of A are (-2u, 0,0). Similarly the coordinates of B and C are (0, -2v, 0) and (0, 0, -2w). x y z + + = 1. The equation of the plane ABC is −2u −2v −2w This plane passes through the point (a, b, c).

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14-26

Analytical Geometry



∴

a b c a b c + + = 1  (i.e.)  + + =2 −u − v −w −2u −2v −2w

The locus of the centre (-u, -v, -w) is

a b c + + = 2. x y z

Example 14.21 Find the centre and radius of the circle x2 + y2 + z2 - 8x + 4y + 8z - 45 = 0, x - 2y + 2z = 3.

Solution The centre of the sphere x2 + y2 + z2 - 8x + 4y + 8z - 45 = 0 is (4, -2, -4).

r = 16 + 4 + 16 + 45 = 81 = 9. N

P 9

C

CN is the perpendicular from the centre of the sphere on the plane x - 2y + 2z = 3. 4+ 4 −8−3 = =1 1+ 4 + 4

∴ NP = CP 2 − CN 2 = 81 − 1 = 80.

Therefore, the radius of the circle is 80 units. The equation of the line CN x−4 y+2 z+4 = = = t. is 1 −2 2 Any point on this line is t + 4, -2t - 2, 2t - 4. This point is the centre of the circle then this lies on the plane x - 2y + 2z - 3 = 0 then t + 4 - 2(2t - 2) + 2(2t - 4) - 3 = 0. 9t - 3 = 0 ∴  t = 1/3.



Therefore, the centre of the circle is

1 2 2   4 + , −2 − , −4 +  3 3 3

M14_ANALYTICALGEOMETRY_ISBN_CH14.indd 26

 13 −8 −10  (i.e.)  , , .  3 3 3 

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Sphere

14-27

Example 14.22 Show that the centres of all sections of the sphere x2 + y2 + z2 = r2 by planes through the point (a , b , g ) lie on the sphere x( x − a ) + y( y − b ) + z ( z − g ) = 0.

Solution Let (x1, y1, z1) be a centre of a section of the sphere x2 + y2 + z2 = r2 by a plane through (a , b , g ) . Then the equation of the plane is x1(x - x1) + y1(y - y1) + z1(z - z1) = 0. This plane passes through the point (a , b , g ) . x1 (a − x1 ) + y1 ( b − y1 ) + z1 (g − z1 ) = 0



Therefore, the locus of (x1, y1, z1) is x(a − x ) + y( b − y ) + z (g − z ) = 0. (i.e.)  x( x − a ) + y( b − y ) + z (g − z ) = 0 which is a sphere.

Example 14.23 Find the equation of the sphere having the circle x2 + y2 + z2 = 5, x -2y + 2z = 5 for a great circle. Find its centre and radius.

Solution Any sphere containing the given circle is x2 + y2 + z2 - 5 + 2l(x - 2y + 2z - 5) = 0. The centre of this sphere is (-l, 2l, -2l). Since the given circle is a great circle, the centre of the sphere should lie on the plane section x- 2y + 2z = 5. −5 -l - 4l - 4l = 5 or l = 9 10 Therefore, the equation of the sphere is x2 + y2 + z2 - 5 (x - 2y + 9 2z - 5) = 0.

9(x2 + y2 - z2 - 5) - 10(x - 2y + 2z - 5) = 0.

 5 −10 10  , . Centre of the sphere is  ,  9 9 9 

r=

25 + 100 + 100 − 45 180 2 5 = = 81 81 3

Example 14.24 Find the equations of the spheres which passes through the circle x2 + y2 + z2 = 5, x + 2y + 3z = 3 and touch the plane 4x + 3y = 15.

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14-28

Analytical Geometry

Solution Any sphere containing the given circle is x2 + y2 + z2 - 5 + l (x + 2y + 3z - 3) = 0. −3l   −l , −l , Centre is  .  2 2 

r=

l2 9l 2 14 l 2 + l2 + + 5 + 5l = + 5l + 5 4 4 4

If the sphere touches the plane 4x + 3y = 15 then the radius of the sphere is equal to the perpendicular distance from the centre on the plane. 7l 2 −2l − 3l − 15 + 5l + 3 = ± 2 4 2 + 32

∴



=±

∴



−5( l + 3) 5

7l 2 + 3l + 3 = ( l + 3)2 2

7l 2 + 6l + 10 = 2( l 2 + 6l + 9)



5l 2 − 6l − 8 = 0 5l 2 − 10 l + 4 l − 8 = 0 5l (l − 2) + 4( l − 2) = 0 ( l − 2)(5l + 4) = 0



−4 5 There are two spheres touching the given plane whose equations are x2 + y2 + 4 z2 - 5 + 2(x + 2y + 3z - 3) = 0 and x2 + y2 + z2 - 5 - (x + 2y + 3z - 3) = 0 5 l = 2,



(i.e.)  x2 + y2 + z2 + 2x + 4y + 6z - 11 = 0



5(x2 + 5(x2 + y2 + z2) - 4x - 8y - 12z - 13 = 0

Example 14.25 Prove that the circles x2 + y2 + z2 - 2x + 3y + 4z - 5 = 0, 5y + 6z + 1 = 0 and x2 + y2 + z2 - 3x - 4y + 5z - 6 = 0, x + 2y - 7z = 0 lie on the same sphere and find its equation.

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Sphere

14-29

Solution The equation of the sphere through the first circle is

x2 + y2 + z2 - 2x + 3y + 4z - 5 + l(5y + 6z + 1) = 0

(14.42)

The equation of the sphere through the second circle is

x2 + y2 + z2 - 3x - 4y + 5z - 6 + m(x + 2y - 7z) = 0

(14.43)

The given circles will lie on the same sphere if equation (14.42) and (14.43) are identical. Therefore, comparing equations (14.42) and (14.43) we get, −2 = −3 + m 3 + 5l = −4 + 2 m 4 + 6l = 5 − 7 m −5 + l = −6 l = −1 m =1

From (14.47), From (14.46),

(14.44) (14.45) (14.46) (14 .447)

These two values l  and m satisfy (14.42), the equations (14.45) and (14.46). Hence, the two given circles lie on the same sphere. The equation of the sphere is x 2 + y 2 + z 2 − 3 x − 4 y + 5 z − 6 + x + 2 y − 7 z = 0.

(i.e.)

x2 + y2 + z2 − 2x − 2 y − 2z − 6 = 0

Example 14.26

x y z + + = 1 meets the circle O, A, B and C. Find the equations of a b c the circumcircle of the triangle ABC and also find its centre.

The plane

Solution

x y z + + = 1. a b c Therefore, the coordinates of A, B and C are (a, 0, 0), (0, b, 0) and (0, 0, c) respectively. Also we know that the equation of the sphere OABC is x2 + y2 + z2 - ax by - cz = 0. Therefore, the equation of the circumcircle of the triangle ABC are x2 + x y z y2 + z2 - ax - by - cz = 0 and + + = 1. a b c The equation of the plane ABC is

M14_ANALYTICALGEOMETRY_ISBN_CH14.indd 29

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14-30

Analytical Geometry

N

P

C

 a b c The centre of the sphere OABC is  , ,  .  2 2 2 b c a y− z− 2 = 2= 2 = t. The equation of the normal CN is 1 1 1 b c a a t b t c t Any point on this line is  + , + , +  .  2 a 2 b 2 c x−

Thus, point lies on the plane

x y z + + = 1. a b c

1a t  1b t 1c t  +  +  +  +  +  =1 a  2 a b  2 b c  2 c



1 1 1  1 ∴t  2 + 2 + 2  = − a 2 b c  1 ∴t = − −2 2( a + b −2 + c −2 )



1  1 1  b c a  a b Hence the centre of the circle is  − , − , − c . 2 2 ∑ a −2 2 2 ∑ a −2 2 2 ∑ a −2     

 a(b −2 + c −2 ) b(c −2 + a −2 ) c( a −2 + b −2  , , (i.e.)   2 ∑ a −2 2 ∑ a −2   2 ∑ a −2

Example 14.27 Obtain the equations to the sphere through the common circle of the sphere x2 + y2 + z2 + 2x + 2y = 0 and the plane x + y + z + 4 = 0 which intersects the plane x + y = 0 in circle of radius 3 units.

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Sphere

14-31

Solution The equation of the sphere containing the given circle is x2 + y2 + z2 + 2x + 2y + l(x + y + z + 4) = 0.   l  l l Centre of this sphere is  − 1 +  , − 1 +  , −  . 2 2 2    2

2

2

 l  l l Radius = 1 +  + 1 +  +   − 4l  2  2  2 =

1 3l 2 − 8l + 8 2 x+y=0 N

P

C s + λp = 0

CN = Perpendicular from the centre C on the plane x + y = 0.



=

 l  l − 1 +  − 1 +   2  2 2



2

1 (2 + l )2 (3l 2 − 8l + 8) = +9 4 2 3l2 - 8l + 8 = 8 + 8l + 2l2 + 36



(i.e.)  l2 - 16l - 36 = 0



(l + 2)(l - 18) = 0



2+l

CP2 = CN2 + NP2



=

∴  l = -2 or 18

Therefore,the equations of the required spheres are x2 + y2 + z2 + 2x + 2y - 2(x + y + z + 4) = 0 and x2 + y2 + z2 + 2x + 2y + 18(x + y + z + 4) = 0.

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14-32

Analytical Geometry



(i.e.)  x2 + y2 + z2 - 2z - 8 = 0



x2 + y2 + z2 + 20x + 20y + 18z + 72 = 0

Example 14.28 Find the equation of the sphere which touches the sphere x2 + y2 + z2 + 2x 6y + 1 = 0 at the point (1, 2, -2) and passes through the origin.

Solution x2 + y2 + z2 + 2x - 6y + 1 = 0



(14.48)

The equation of the tangent plane at (1, 2, -2) is x + 2y - 2z + (x + 1) 3(y + 2) + 1 = 0. 2x - y - 2z - 4 = 0



(14.49)

The equation of the sphere passing through the intersection of (14.48) and (14.49) is x2 + y2 + z2 + 2x - 6y + 1 + l(2x - y - 2z - 4) = 0. This sphere passes through the origin. 1 ∴  1 - 4l = 0 or l = 4 Therefore, the equation of the required sphere is 4(x2 + y2 + z2 + 2x 6y + 1) + (2x - y - 2z - 4) = 0. (i.e.)  4(x2 + y2 + z2) + 10x -25y - 2z = 0



Example 14.29 Show that the condition for the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 to cut the sphere x2 + y2 + z2 + 2u1x + 2v1y + 2w1z + d1= 0 in a great circle is 2uu1 + 2vv1 + 2ww1 - (d + d1) = 2r12 where r1 is the radius of the latter sphere.

Solution

S: x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0



S1: x2 + y2 + z2 + 2u1x + 2v1y + 2w1z + d1 = 0

The intersection of these two sphere is S - S1 = 0.

(i.e.)  2(u - u1)x + 2(v - v1)y + 2(w - w1)z + d - d1 = 0.

The centre of the sphere S1 = 0 is (-u1, -v1, -w1).

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Sphere

14-33

Since S1 = 0 cuts S2 = 0 in a great circle, the centre of the sphere lies on the plane of intersection S1 - S2 = 0.

∴  2(u - u1)(-u1) + 2(v - v1)(-v1) + 2(w - w1)(-w1) + d - d1 = 0



(i.e.)  -2uu1 - 2vv1 - 2ww1 + 2(u12 + v12 + w12) + d - d1 = 0



2uu1 + 2vv1 + 2ww1 - (d + d1) = 2(u12 + v12 + w12 - d1)



(i.e.)  2uu1 + 2vv1 + 2ww1 - (d + d1) = 2r12

Example 14.30 A tangent plane to the sphere x2 + y2 + z2 = r2 makes intercepts a, b and c on the coordinate axes. Prove that a-2 + b-2 + c-2 = r-2.

Solution Let P(x1, y1, z1) be a point on the sphere x2 + y2 + z2 = r2. ∴ x12 + y12 + z12 = r 2



(14.50)

The equation of the tangent plane at P is xx1 + yy1 + zz1 = r2.

xx1 yy1 zz1 + 2 + 2 =1 r2 r r

(i.e.) 

Therefore, the intercepts made by the plane on the coordinate axes are r2 r2 r2 a= , b= , c= . x1 y1 z1

1 1 1 x12 + y12 + z12 r 2 + + = = 4 from (14.50) a2 b2 c 2 r4 r =



1 r2

∴  a-2 + b-2 + c-2 = r -2

Example 14.31 Two spheres of radii r1 and r2 intersect orthogonally. Prove that the radius of r1r2 the common circle is . r12 + r12

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14-34

Analytical Geometry

Solution Let the equation of the common circle be x2 + y2 = r2, z = 0



(14.51)

Then the equation of the sphere through the given circle is x + y + z2 r2 + l z = 0 where l is arbitrary. Let the equation of the two spheres through the given circle be x2 + y2 + z2 r2 + l1z = 0 and x2 + y2 + z2 - r2 + l2z = 0 If r1 and r2 are the radii of the above two spheres then 2



r1 =

l 12 1 l 12 + 4 r 2 + r2 = 4 2

l 12 1 l 22 + 4 r 2 + r2 = 4 2 Since the two spheres cut orthogonally.

r2 =



2uu1 + 2vv1 + 2ww1 = d + d1



∴ 2



2

l1 l2 = −r 2 − r 2 2 2 l1l2 = -4r2

(14.52)

(14.53)

Eliminating l1 and l2 from (14.52) and (14.53), we get

4 r12 − 4 r 2 4 r22 − 4 r 2 = −4 r 2



16( r12 − r 2 )( r22 − r 2 ) = 16r 4



r12 r22 − r 2 ( r12 + r22 ) + r 4 = r 4 ∴r =



r1r2 r12 + r22

Example 14.32 Find the equation of the sphere which touches the plane 3x + 2y - z + 2 = 0 at the point (1, -2, 1) and cuts orthogonally the sphere x2 + y2 + z2 - 4x + 6y + 4 = 0.

Solution Let the equation of the required sphere be

x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0

M14_ANALYTICALGEOMETRY_ISBN_CH14.indd 34

(14.54)

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Sphere

14-35

This sphere touches the plane 3x + 2y - z + 2 = 0 at (1, -2, 1). The equation of the tangent plane at (1, -2, 1) is xx1 + yy1 + zz1 + u(x + x1) + v(y + y1) + w(z + z1) + d =0.

(i.e.)  x(u + 1) + y(v - 2) + z(w + 1) + u - 2v + w + d = 0 (14.55)

But the tangent plane is given as 3x + 2y - z + 2 = 0



(14.56)

Identifying equations (14.55) and (14.56) we get,

u + 1 v − 2 w + 1 u − 2v + w + d = = = = k (say ) −1 3 2 2



∴  u = 3k - 1

(14.57)



v = 2k + 2

(14.58)

w = -k - 1 u - 2v + w + d = 2k (3k - 1) - 2(2k + 2) + (-k - 1) + d = 2k d = 4k + 6

(14.59)

(14.60)

The sphere (14.54) cuts orthogonally the sphere

x2 + y2 + z2 - 4x + 6y + 4 = 0



∴  2u(-2) + 2v(3) + d + 4 = 1



-4u + 6v + d + 4 = 1



-4(3k - 1) + 6(2k + 2) + 4k + 6 + 4 = 1

(14.61)



-12k + 4 + 12k + 12 = 4k + 10



16 = 4k + 10



3 7 7 ∴ u = 3k − 1 = 2 v = 2k + 2 = 5



w = −k − 1 = −



d = 4 k + 6 = 12



6 = 4k or k =

5 2

Therefore, the equation of the sphere is x2 + y2 + z2 + 7x + 10y - 5z + 12 = 0.

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14-36

Analytical Geometry

Example 14.33 Find the equations of the radical planes of the spheres x2 + y2 + z2 + 2x + 2y + 2z + 2 = 0, x2 + y2 + z2 + 4y = 0 and x2 + y2 + z2 + 3x - 2y + 8z + 6 = 0. Also find the radical line and the radical centre.

Solution Consider the equations,

x2 + y2 + z2 + 2x + 2y + 2z + 2 = 0 x2 + y2 + z2 + 4y = 0 2 2 2 x + y + z + 3x - 2y + 8z + 6 = 0

(14.62) (14.63) (14.64)

The radical plane of the spheres (14.62) and (14.63) is S1 - S 2 = 0. (i.e.)  2x - 2y + 2z + 2 = 0 x-y+z+1=0 The radical plane of the spheres (14.63) and (14.64) is S2 - S3 = 0.

(i.e.)  3x - 6y - 8z + 6 = 0

The radical plane of the sphere (14.62) and S3 is S1 - S3 = 0.

(i.e.)  x - 4y + 6z + 4 = 0

The equation of the radical line of the spheres are given by

S1 - S 2 = 0, S 2 - S 3 = 0 or  x - y + z + 1 = 0, 3x - 6y + 8z + 6 = 0

Also the radical line is given by

3x - 6y + 8z + 6 = 0, 2x - 3y + 7z + 4 = 0.

The radical centre is the point of intersection of the above two lines. So we have to solve the equations

x - y + z = -1 3x - 6y + 8z = 6 2x - 3y + 7z = -4

Solving these equations we get

x=

1 −3 −1 ,y = ,z = 6 2 10

 −1 1 −3  Therefore, the radical centre is  , ,  .  6 2 10 

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Sphere

14-37

Example 14.34 Find the equation of the sphere through the origin and coaxal with the spheres x2 + y2 + z2 = 1 and x2 + y2 + z2 + x + 2y + 3z - 5 = 0.

Solution The radical plane of the two given spheres is S - S1 = 0. (i.e.)  x + 2y + 3z - 4 = 0



The equation of any sphere coaxal with given spheres is S + lP = 0.

(i.e.)  x2 + y2 + z2 - 1 + l(x + 2y + 3z - 4) = 0

This sphere passes through the origin.

∴  -1 - 4l = 0



∴  l = −

1 4

Therefore, the equation of the required sphere is x2 + y2 + z2 1 1 − ( x + 2 y + 3 z − 4) = 0 . 4

(i.e.)  4(x2 + y2 + z2) - 4 - x - 2y - 3z + 4 = 0

\ 4(x2 + y2 + z2) - x - 2y - 3z = 0

Example 14.35 Find the limiting points of the coaxal system of spheres determined by x2 + y2 + z2 + 4x - 2y + 2z + 6 = 0 and x2 + y2 + z2 + 2x - 4y - 2z + 6 = 0.

Solution The radical plane of the two given spheres is 2x + 2y + 4z = 0. The equation to any sphere of the coaxal system is x2 + y2 + z2 + 4x - 2y + 2z + 6 + l(x + y + 2z) = 0. l l   The centre is  −2 − ,1 − , −1 − l  .   2 2

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14-38

Analytical Geometry

2

2

l  l  Radius is  2 +  + 1 −  + (1 + l )2 − 6 .  2  2 For limiting point of the coaxal system radius = 0.

2 ( 4 + l )2 (2 − l )2 + + (1 + l ) − 6 = 0 4 4



(4 + l)2 + (2 - l)2 + 4(1 + l)2 - 24 = 0



6l2 + 12l = 0



6l (l + 2) = 0 l = 0, l = -2

Therefore, the limiting points are the centres of point spheres of the coaxal system. Therefore, the limiting points are (-2, 1, -1) and (-1, 2, 1).

Example 14.36 The point (-1, 2, 1) is a limiting point of a coaxal system of spheres of which x2 + y2 + z2 + 3x -2y + 6 = 0 is a member. Find the equation of the radical plane of this system and the coordinates of other limiting point.

Solution The point gives belonging to the coaxal system corresponding to the limiting point (-1, 2, 1) is (x + 1)2 + (y - 2)2 + (z - 1)2 = 0.

(i.e.)  x2 + y2 + z2 + 2x - 4y - 2z + 6 = 0

Two members of the system of the system are x2 + y2 + 3x - 3y + 6 = 0 and x2 + y2 + z2 + 2x - 4y - 2z + 6 = 0. The radical plane of the coaxal system is x + y + 2z = 0. Any member of the system is x2 + y2 + 2x - 4y - 2z + 6 + l(x + y + 2z) = 0. l l   Centre is  −1 − , 2 − ,1 − l    2 2



r=

(2 + l )2 ( 4 − l )2 + + (1 − l )2 − 6 4 4

For limiting points radius = 0

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Sphere



(2 + l)2 + (4 - l)2 + 4(1 - l)2 - 24 = 0



6l2 - 12l = 0 \ l = 0, l = 2

14-39

When l = 0 centre is (-1, 2, 1) which is the given limiting point. When l = 2, the centre is (-2, 1, -1) which is other limiting point.

Example 14.37 Show that the spheres x2 + y2 + z2 = 25 and x2 + y2 + z2 -24x - 40y - 18z + 225 = 0 touch externally. Find their point of contact.

Solution

x2 + y2 + z2 = 25

(14.65)



x2 + y2 + z2 - 24x - 40y - 18z + 225 = 0

(14.66)

The centre and radius of sphere (14.65) are C1(0, 0, 0), r1 = 5



Centre and radius of the sphere (14.66) are

C2(12, 20, 9) and r = 144 + 400 + 81 − 225 = 20

The distance between the centres

C1C2 = 144 + 400 + 81 = 625 = 25 ∴ r1 + r2 = C1C2

Hence the two given spheres touch externally. 20



C2 (12, 20, 9)

4

r2

5 r 1 1

C1 (0, 0, 0)

Therefore, the point of contact divides the lines of centres in the ratio 4 : 1 Therefore, the coordinates of the point of contact is



 4 × 0 + 1 × 12 4 × 0 + 1 × 20 4 × 0 + 1 × 9  , ,   5 5 5 9  12 (i.e.)  , 4,   5 5

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14-40

Analytical Geometry

Exercises 1 1.

Find the equation of the sphere with (i) centre at (1, −2, 3) and radius 5 units.  1 2 1 (ii) centre at  − , ,  and radius 1 unit.  3 3 3

  

(iii) centre at (1, 2, 3) and radius 4 units. Ans.:  (i) x2 + y2 + z2 - 2x + 4y - 6z - 11 = 0 (ii) 3(x2 + y2 + z2) + 2x - 4y - 2z - 1 = 0 (iii) x2 + y2 + z2 - 2x - 4y - 6z - 2 = 0

2. 

Find the coordinates of the centre and radius of the following spheres: (i) x2 + y2 + z2 + 2x - 4y - 6z + 15 = 0 (ii) 2x2 + 2y2 + 2z2 - 2x - 4y - 6z - 1 = 0 (iii) ax2 + ay2 + az2 + 2ux + 2vy + 2wz + d = 0 Ans.:  (i) (−1, 2, 3), r = 3 3 1 (ii)  , −1,  , r = 2 2 2



 −u − v −w  (iii)  , , ,  a a a 

 3.

u 2 + v 2 + w 2 − ad a2



Find the equation of the sphere whose centre is at the point (1, 2, 3) and which passes through the point (3, 1, 2). Ans.:  x2 + y2 + z2 - 2x - 4y - 6z + 8 = 0

4.    

Find the equation of the sphere passing through points: (i) (0, 0, 0), (0, 1, −1), (−1, 2, 0) and (1, 2, 3) (ii) (2, 0, 1), (1 - 5, –1), (0, −2, 3) and (4, −1, 3) (iii) (0, −1, 2), (0, −2, 3), (1, 5, −1) and (2, 0, 1) (iv) (−1, 1, 1), (1, −1, 1), (1, 1, −1), (0, 0, 0) Ans.:   (i)  (x2 + y2 + z2) - 13x - 25y - 11z = 0 (ii)  x2 + y2 + z2 - 4x + 10y - 2z + 5 = 0 (iii)  x2 + y2 + z2 - 4x - 14y - 22z + 25 = 0 (iv)  x2 + y2 + z2 − 3x - 3y - 3z = 0

5.

Find the equation of the sphere on the line joining the points (2, −3, –1) and (1, −2, −1) at the ends of a diameter. Ans.:  x2 + y2 + z2 - 3x + 5y + 7 = 0

 6.  7. 

Find the radius of the sphere touching the plane 2x + 2y - z - 1 = 0 and concentric with the sphere 2x2 + 2y2 + 2z2 + x + 2y - z = 0. 7 Ans.: units 12 Find the equation of the sphere passing through the points (0, 2, 3), (1, 1, −1), (-5, 4, 2) and having its centre on the plane 3x + 4y + 2z - 6 = 0. Ans.:  9(x2 + y2 + z2) + 28x + 7y - 20z - 96 = 0

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Sphere

8.

Prove that a sphere can be made to pass through the midpoints of the edges of x y z a tetrahedron whose faces are x = 0, y = 0, z = 0 and + + = 2. Find its a b c equation.

Ans.:  x2 + y2 + z2 - ax - by - cz = 0

 9. 

14-41

Find the condition that the plane lx + my + nz = p may touch the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. Ans.:  (ul + vm + wn + p)2 = (l 2 + m2 + n2) (u2 + v2 + w2 – d)

10.

Prove that the sphere circumscribing the tetrahedron whose faces are y + z = 0, z + x = 0, x + y = 0 and x + y + z = 1 is x2 + y2 + z2 - 3(x + y + z) = 0.

11.

A point moves such that, the sum of the squares of its distances from the six faces of a cube is a constant. Prove that its locus is the sphere x2 + y2 + z2 = 3(k2 - a2).

12.

Prove that the spheres x2 + y2 + z2 = 100 and x2 + y2 + z2 −12x + 4y - 6z + 40 = 0 touch internally and find the point of contact.  60 −20 30  , Ans.:   ,  2 7 7 

 13.  14.  15.  16.  17.

Prove that the spheres x2 + y2 + z2 = 25 and x2 + y2 + z2 - 24x - 40y - 18z + 225 = 0 touch externally. Find the point of contact.  9 12  Ans.:   , , 4 5 5  Find the condition that the plane lx + my + nz = p to be a tangent to the sphere x2 + y2 + z2 = r.2 Ans.:  r2 (l2 + m2 + n2) = p2 Find the equation of the sphere which touches the coordinate planes and whose centre lies in the first octant. Ans.:  x2 + y2 + z2 - 2vx - 2vy −2vz + 2v2 = 0 Find the equation of the sphere with centre at (1, −1, 2) and touching the plane 2x - 2y + z =3. Ans.: x2 + y2 + z2 - 2x + 2y + z + 5 = 0 Find the equation of the sphere which has the points (2, 7, -4) and (4, 5, −1) as the extremities of a diameter. Ans.:  x2 + y2 + z2 – 6x - 12y + 5z + 47 = 0

 18.  19.

Find the equation of the sphere which touches the three coordinate planes and the plane 2x + y + 2z = 6 and being in the first octant. Ans.:  x2 + y2 + z2 – 6x - 6y - 6z + 18 = 0 A point P moves from two points A(1, 3, 4) and B(1, −2, −1) such that 3.PA = 2.PB. Show that the locus of P is the sphere x2 + y2 + z2 - 2x - 4y − 16z + 42 = 0. Show also that this sphere divides A and B internally and externally in the ration 2:3.

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Analytical Geometry

20.

A plane passes through a fixed point (a, b, c). Show that the locus of the foot of the perpendicular to it from the origin is the sphere x2 + y2 + z2 - ax by - cz = 0.

21.



A variable sphere passes through the origin O and meets the coordinate axes in A, B and C so that the volume of the tetrahedron OABC is a constant. Find the locus of the centre of the sphere. Ans.:  xyz = k2

22.

Find the equation of the sphere on the line joining the points:

 

(i) (4, −1, 2) and (2, 3, 6) as the extremities of a diameter (ii) (2, −3, 4) and (-5, 6, -7) as the extremities of a diameter Ans.:  x2 + y2 + z2 - 6x - 2y - 8z + 17 = 0 x2 + y2 + z2 + 3x - 3y + 3z - 56 = 0

23.

A plane passes through a fixed point (a, b, c) and cuts the axes in A, B and C. a b c Show that the locus of centre of the sphere ABC is + + = 2. x y z

24.

A sphere of constant radius 2k passes through the origin and meets the axes in A, B and C. Prove that the locus of the centroid of ∆ABC is 9(x2 + y2 + z2) = a2.

25.

The tangent plane at any point of the sphere x2 + y2 + z2 = a2 meets the coordinate axes at A, B and C. Find the locus of the point of intersection of the planes drawn parallel to the coordinate planes through A, B and C. Ans.:  x−2 + y−2 + z−2 = a−2

 26.

OA, OB and OC are three mutually perpendicular lines through the origin and their direction cosines are l1, m, n; l2, m2, n2 and l3, m3, n3. If OA = a, OB = b, OC = c then prove that the equation of the sphere OABC is x2 + y2 + z2 x(al1 + bl2 + cl3) - y(am1 + bm2 + cm3) - z(an1 + bn2 + cn3) = 0.

Exercises 2 1.

Find the centre and radius of a section of the sphere x2 + y2 + z2 = 1 by the plane lx + my + nz = 1.  l m n  l 2 + m 2 + n2 − 1 Ans.:  , , , . 2 2 2 l 2 + m 2 + n2  ∑l ∑l ∑l 

2.

Find the equation of the sphere through the circle x2 + y2 + z2 = 5, x + 2y + 3z = 3 and the point (1, 2, 3). Ans.: 5(x2 + y2 + z2) - 4x - 8y - 12z -13 = 0.

3.

Prove that the plane x + 2y - z = 4 cuts the sphere x2 + y2 + z2 - x + z - 2 = 0 in a circle of radius unity and find the equation of the sphere which has this circle for one of its great circles. Ans.: x2 + y2 + z2 - 2x - 2y + 2z - 2 = 0

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4.

Find the centre and radius of the circle in which the sphere x2 + y2 + z2 = 25 is cut by the plane 2x + y + 2z = 9. Ans.: (2, 1, 2); 4

5.

Show that the intersection of the sphere x2 + y2 + z2 - 2x - 4y – 6z – 2 = 0 and the plane x -2y + 2z - 20 = 0 is a circle of radius 7 with its centre at (2, 4, 5).

6.

Find the centre and radius of the circle x2 + y2 + z2 - 2x - 4z + 1 = 0, x + 2y + 2z = 11.  5 19 28  11 Ans.:  , ,  , 9 9 9  3

7.

Prove that the radius of the circle x2 + y2 + z2 + x + y + z = 4, x + y + z = 0 is 2.

8.

Find the centre and radius of the circle x2 + y2 + z2 + 2x - 2y - 4z - 19 = 0, x + 2y+ 2z + 7 = 0.  −7 −5 −2  Ans.:  , ,  , 5  3 3 3

9.

Find the radius of the circle 3x2 + 3y2 + 3z2 + x - 5y - 2 = 0, x + y = 2.

10.

Show the two circles 2(x2 + y2 + z2) + 8x + 13y + 17z - 17 = 0, 2x + y - 3z + 1 = 0 and x2 + y2 + z2 + 3x - 4y + 3z = 0, x - y + 2z - 4 = 0 lie on the same sphere and find its equation. Ans.: x2 + y2 + z2 + 5x - 6y + 7z - 8 = 0

11.

Prove that the two circles x2 + y2 + z2 - 2x + 3y + 4z - 5 = 0, 5y + 6z + 1 = 0 and x2 + y2 + z2 - 3x - 4y + 5z - 6 = 0, x + 2y - 7 = 0 lie on the same sphere and find its equation. Ans.: x2 + y2 + z2 - 2x - 2y - 2z - 6 = 0

12.

Find the area of the section of the sphere x2 + y2 + z2 + 12x - 2y - 6z + 30 = 0 by the plane x - y + 2z + 5 = 0. 49 Ans.: p 8

13.

Find the equation of the sphere which has its centre on the plane 5x + y - 4z + 3 = 0 and passing through the circle x2 + y2 + z2 - 3x + 4y - 2z + 8 = 0, 4x 5y + 3z - 3 = 0. Ans.: x2 + y2 + z2 + 9x - 11y + 7z - 1 = 0

14.

Find the equation of the sphere having the circle x2 + y2 + z2 + 10x - 4z - 8 = 0, x + y + z - 3 = 0 as a great circle. Ans.: x2 + y2 + z2 + 6x - 4y - 3z + 4 = 0

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Analytical Geometry

x y z + + = 0 and meets the axes a b c at A, B and C. Prove that the circle ABC lies on the surface  b c  c a  a b yz  +  + zx  +  + xy  +  = 0  c b  a c  b a

15.

A variable plane is parallel to a given plane

16.

Find the equation of the spheres which pass through the circle x2 + y2 + z2 – 4x - y + 6z + 12 = 0, 2x + 3y -7z = 10 and touch the plane x - 2y - 2z = 1. Ans.: x2 + y2 + z2 - 2x + 2y – 4z + 2 = 0 x2 + y2 + z2 – 6x – 4y + 10z + 22 = 0

17.

Find the equation of the sphere which pass through the circle x2 + y2 + z2 = 5, x + 2y + 3z = 5 and touch the plane z = 0. Ans.: x2 + y2 + z2 - 2x + y + 5z + 5 = 0 5(x2 + y2 + z2) - (2x - 4y + 5z + 1 = 0)

18.

Find the centre and radius of the circle x2 + y2 + z2 - 2x + 4y + 2z - 6 =0, x + 2y + 2z - 4 = 0. Ans.: (2, 0, 1), 3

19.

Find the equation of the sphere which passes through the point (3, 1, 2) and meets XOY plane in a circle of radius 3 units with the centre at the point (1, -2, 0). Ans.: x2 + y2 + z2 - 2x + 4y - 4z - 4 = 0

20.

Find the centre and radius of the circle x2 + y2 + z2 + 12x - 12y - 16z + 111 = 0, 2x + 2y + z = 17. Ans.: (-4, 8, 9), r = 4

21.

Find the centre and radius of the circle x2 + y2 + z2 + 2x + 2y - 4z - 19 = 0, x + 2y + 2z + 7 = 0.  −7 −10 −4  Ans.:  , , ,3  3 3 3 

22.

Find the centre and radius of the circle x2 + y2 + z2 = 9, x + y + z = 1.  1 1 1 Ans.:  , ,  ,  3 3 3

26 3

23.

Find the equation of the sphere through the circle x2 + y2 + z2 = 9, 2x + 3y + 4z = 5 and the point (1, 2, 1). Ans.: 3(x2 + y2 + z2) - 2x - 2y - 4z - 22 = 0

24.

Find the equation of the sphere containing the circle x2 + y2 + z2 - 2x = 9, z = 0 and the point (4, 5, 6). Ans.: x2 + y2 + z2 - 2x - 10z - 9 = 0

25.

Find the equation of the sphere passing through the circle x2 + y2 = a2, z = 0 and the point (a, b, l ). Ans.: r(x2 + y2 + z2 - a2) - z(a 2 + b  2 + g  2 - a2) = 0

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Sphere

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26.

Find the equation of the sphere through the circle x2 + y2 + z2 + 2x + 3y + 6 = 0, x - 2y + 4z - 9 = 0 and the centre of the sphere. Ans.: x2 + y2 + z2 – 2x + 4y - 6z + 5 = 0

27.

Find the equations of the sphere through the circle x2 + y2 + z2 = 1, 2x + 4y + 5z = 6 and touching the plane z = 0. Ans.: x2 + y2 + z2 - 2x - 4y - 5z + 5 = 0 5(x2 + y2 + z2) - 2x - 4y - 5z + 1 = 0

28.

Find the equation of the sphere having the circle x2 + y2 + z2 + 10y - 4z - 8 = 0, x + y + 23 = 0 as a great circle. Ans.: x2 + y2 + z2 - 4x + 6y - 8z + 4 = 0

29.

Show that the two circles x2 + y2 + z2 - y + 2z = 0, x - y + z – 2 = 0 and x2 + y2 + z2 + x - 3y + z - 5 = 0, 2x - y + 4z -1 = 0 lie on the same sphere and find its equation. Ans.: x2 + y2 + z2 + 3x - 4y + 5z - 6 = 0

30.

Prove that the circles x2 + y2 + z2 - 2x + 3y + 4z - 5 = 0, 5y + 6z + 1 = 0 and x2 + y2 + z2 - 3x + 4y + 5z - 6 = 0, x + 2y - 7z = 0 lie on the same sphere. Find its equation. Ans.: x2 + y2 + z2 - 2x - 2y - 2z - 6 = 0

31.

Find the conditions that the circles x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0, lx + my + nz = p and (x2 + y2 + z2) 2u′x + 2v′y + 2w′z + d′  = 0, l′x + m′y + n′z = p′   to lie on the same circle. 2(u − u ′ ) 2( v − v ′ ) 2(w − w ′ ) d − d ′ l m n −p = 0 Ans.: ∆ l′ m′ n′ − p′

32.

Find the centre and radius of the circle formed by the intersection of the sphere x2 + y2 + z2 = 2225 and the plane 2x - 2y + z = 27. Ans.: (6, -6, 3), 12

33.

Find the centre and radius of the circle x2 + y2 + z2 = 25, x + 2y + 2z = 9. Ans.: (1, 2, 2), 4

34.

Find the equation of the circle which lies on the sphere x2 + y2 + z2 = 25 and has the centre at (1, 2, 3). Ans.: x2 + y2 + z2 = 25, x + 2y + 3z = 14

35.

A plane passes through a point (a, b, g ) and intersects the sphere x2 + y2 + z2 = a2. Show that the locus of the centre of the circle of intersection is the sphere x(x - a ) + y(y - b ) + z(z - g ) = 0.

36.

Find the equation of the sphere through the circle x2 + y2 + z2 - 4 = 0 and the point (2, 1, 1). Ans.: x2 + y2 + z2 - 2x + y - 2z - 1 = 0

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14-46

37.

Analytical Geometry

Find the equation of the sphere through the circle x2 + y2 + z2 = 9, 2x + 3y + 4z = 5 and through the origin. Ans.: 5(x2 + y2 + z2) - 18x - 27y - 36z = 0

38.

Show that the two circles 2(x2 + y2 + z2) + 8x - 13y + 17z - 17 = 0, 2x + y 3z + 1 = 0 and x2 + y2 + z2 + 3x - 4y + 3z = 0, x - y + 2z - 4 = 0 lie on the same sphere and find its equation. Ans.: x2 + y2 + z2 + 5x - 6y - 7z - 8 = 0

39.

Find the equation of the sphere which has its centre on the plane 5x + y - 4z + 3 = 0 and passing through the circle x2 + y2 + z2 – 3x + 4y – 2z + 8 = 0, 4x – 5y + 3z – 3 = 0. Ans.: x2 + y2 + z2 + 9x - 11y + 7z - 1 = 0

40.

Find the equation of the sphere which has the circle S = x2 + y2 + z2 + 2x + 4y + 6z - 11 = 0, 2x + y + 2z + 1 = 0 as great circle. Ans.: x2 + y2 + z2 - 2x + 2y + 2z - 13 = 0.

41.

Find the equation of the sphere whose radius is 1 and which passes through the circle of intersection of the spheres x2 + y2 + z2 + 2x + 2y + 2z - 6 = 0 and x2 + y2 + z2 + 3x + 3y - z - 1 = 0. Ans.: 3x2 + 3y2 + 3z2 + 16x + 16y + 4z + 32 = 0

42.

If r is the radius of the circle x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0, lx + my + nz = 0 then prove that (r2 + d2) (l2 + m2 + n2) = (mw – nv)2 + (nv – lw)2 + (lv – mu)2. Find the equation of the sphere through the circle x2 + y2 = 4, z = 0 meeting the plane x + 2y + 2z = 0 in a circle of radius 3. Ans.: x2 + y2 + z2 – 6z – 4 = 0

43.

44.

Find the equation of the sphere through the circle x2 + y2 + z2 = 1, 2x + 3y + 4z = 5 and which intersect the sphere x2 + y2 + z2 + 3(x – y + z) – 56 = 0 orthogonally, x2 + y2 + z2 – 12x – 18y – 24z + 29 = 0.

45.

The plane ABC whose equation is

x y z + + = 1 meets the axes in A, B and C. a b c Find the equation to determine the circumcircle of the triangle ABC and obtain the coordinates of its centre. x y z Ans.: x2 + y2 + z2 - ax - by - cz = 0, + + = 1 a b c  a(b −2 + c −2 ) b(c −2 + a −2 ) c(c −2 + b −2 )   2 ∑ a −2 , 2 ∑ a −2 , 2 ∑ a −2 

46.

Find the equation of the circle circumscribing the triangle formed by the three points (a, 0, 0), (0, b, 0), (0, 0, c). Obtain the coordinates of the centre of the circle. Ans.: x2 + y2 + z2 – x – 2y – 3z = 0, 6x – 3y – 2z – 6 = 0  −13 40 135  Centre =  , ,  98 49 98 

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Sphere

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47.

Find the equation of the sphere through the circle x2 + y2 + z2 + 2x + 3y + 6 = 0, x – 2y + 4z – 9 = 0 and the centre of the sphere x2 + y2 + z2 – 2x + 4y – 6z + 5 = 0. Ans.: x2 + y2 + z2 + 7y – 8z + 24 = 0

48.

Find the equation of the sphere having its centre on the plane 4x – 5y – z – 3 = 0 and passing through the circle x2 + y2 + z2 – 2x – 3y + 4z + 8 = 0, x2 + y2 + z2 + 4x + 5y – 6z + 2 = 0. Ans.: x2 + y2 + z2 + 7x + 9y – 11z – 1 = 0

49.

A circle with centre (2, 3, 0) and radius unity is drawn on the plane z = 0. Find the equation of the sphere which passes through the circle and the point (1, 1, 1). Ans.: x2 + y2 + z2 – 4x – 6y – 6z + 12 = 0

50.

Find the equation of the sphere which passes through the circle x2 + y2 = 4, z = 0 and is cut by the plane x + 2y + 2z = 0 in a circle of radius 3. Ans.: x2 + y2 + z2 + 6z – 4 = 0 x2 + y2 + z2 – 6z – 4 = 0

51.

Prove that the plane x + 2y – z – 4 = 0 cuts the sphere x2 + y2 + z2 – x + z – 2 = 0 in a circle of radius unity and also find the equation of the sphere which has this circle as great circle. Ans.: x2 + y2 + z2 – 2x – 2y + 2z + 2 = 0

52.

Find the equation of the sphere having the circle x2 + y2 + z2 + 10y – 4z – 3 = 0, x + y + z – 3 = 0 as a great circle. Ans.: x2 + y2 + z2 – 4x + 6y – 8z + 4 = 0

53.

P is a variable point on a given line and A, B and C are projections on the axes. Show that the sphere OABC passes through a fixed circle.

Exercises 3 1.

Find the equations of the spheres which pass through the circle x2 + y2 + z2 = 5, x + 2y + 3z = 5 and touch the plane z = 0. Ans.: x2 + y2 + z2 – 2x + y + 5z + 5 = 0 5(x2 + y2 + z2) – 2x – 4y + 5z + 1 = 0

2.

Find the equations of the spheres which pass through the circle x2 + y2 + z2 – 4x – y + 3z + 12 = 0, 2x + 3y – 8z = 10 and touch the plane x – 2y – 2z = 1. Ans.: x2 + y2 + z2 – 2x + 2y – 4z + 2 = 0 x2 + y2 + z2 – 6x – 4 y + 10z + 22 = 0

3.

Show that the tangent plane at (1, 2, 3) to the sphere x2 + y2 + z2 + x + y + z – 20 = 0 is 3x + 5y + 7z – 34 = 0.

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Analytical Geometry

4.

Find the equation of the sphere which touches the sphere x2 + y2 + z2 – x + 3y + 2z – 3 = 0 at (1, 1, –1) and passes through the origin. Ans.: 2x2 + 2y2 + 2z2 – 3x + y + 4z = 0

5.

Find the equation of the sphere which touches the sphere x2 + y2 + z2 + 2x – 6y + 1 = 0 at (1, 2, –2) and passes through the origin. Ans.: 4(x2 + y2 + z2) + 10x – 28y – 2z = 0

6.

Show that the plane 2x – 2y + z + 16 = 0 touches the sphere x2 + y2 + z2 + 2x + 4y + 2z – 3 = 0 and find the point of contact. Ans.: (–3, 4, –2)

7.

Find the equation of the tangent plane at the origin to the sphere x2 + y2 + z2 – 8x – 6y + 4z = 0. Ans.: 4x – 3y + 2z = 0

8.

Find the equation of the tangent planes to the sphere x2 + y2 + z2 + 2x – 4y + 6z – 7 = 0 which passes through the line 6x – 3y – 23 = 0, 3z + 2 = 0. Ans.: 8x – 4y + z – 34 = 0, 4x – 2y – z – 16 = 0

9.

Show that the plane 2x – 2y + z – 12 = 0 touches the sphere x2 + y2 + z2 – 2x – 4y + 2z – 3 = 0.

10.

Show that the point P(1, –3, 1) lies on the sphere x2 + y2 + z2 + 2x + 2y – 7 = 0 and obtain the equation of the tangent plane at P. Ans.: 2x – 2y + z = 9

11.

If the point (5, 1, 4) is one extremities of a diameter of the sphere x2 + y2 + z2 – 2x – 2y – 2z – 22 = 0 and find the coordinates of the other extremity. Find the equation to the tangent planes at the two extremities and show that they are parallel. Ans.: (–3, 1, –2); 4x + 3y – 22 = 0, 4x + 3y + 8 = 0

12.

Find the value of k for which the plane x + y + z = k touches the sphere x2 + y2 + z2 – 2x – 4y – 6z + 11 = 0. Find the point of contact in each case. Ans.: k = 3 or 9; (0, 1, 2), (2, 3, 4)

13.

Find the equation to the tangent planes to the sphere x2 + y2 + z2 – 2x – 4y – 6z – 2 = 0 which are parallel to the plane x + 2y + 2z – 20 = 0. Ans.: x + 2y + 2z – 23 = 0 x + 2y + 2z – 1 = 0

14.

A sphere touches the plane x – 2y – 2z – 7 = 0 at the point (3, –1, –1) and passes through the point (1, 1, –3). Find the equation. Ans.: x2 + y2 + z2 – 10y – 10z – 31 = 0

15.

Show that the line

x−7 y−4 z+3 = = touches the sphere x2 + y2 + z2 – 6x + 2 7 10 2y – 4z + 5 = 0. Find the point of contact. Ans.: (5, –3, 3)

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Sphere

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16.

Tangent planes at any point of the sphere x2 + y2 + z2 = r2 meets the coordinate axes at A, B and C. Show that the locus of the point of intersection of the planes drawn parallel to the coordinate planes through the points A, B and C is the surface x−2 + y−2 + z-2 = r-2.

17.

Find the condition that the line

18.

Find the equation of the sphere which touches the sphere x2 + y2 + z2 + 3y – x + 2z – 3 = 0 at (1, 1, –1) and passes through the origin. Ans.: 2x2 + 2y2 + 2z2 – 3x + y + 4z = 0

19.

Find the equation of the tangent line in symmetrical form to the circle x2 + y2 + z2 + 5x – 7y + 2z – 8 = 0, 3x – 2y + 4z + 3 = 0. x+3 y −5 z −7 = = Ans.: 32 34 −7

20.

Show that the plane 2x – 2y – z + 12 = 0 touches the sphere x2 + y2 + z2 – 2x – 4y + 2z – 3 = 0 and find the point of contact. Ans.: (–1, 4, –2)

21.

Find the equation of the sphere which touches the sphere x2 + y2 + z2 + 2x – 6y + 1 = 0 at the point (1, 2, –2) and passes through the origin. Ans.: 4(x2 + y2 + z2) + 10x – 25y – 22 = 0

22.

Find the equations of the spheres which pass through the circle x2 + y2 + z2 = 1, 2x + 4y + 5z – 6 = 0 and touch the plane z = 0. Ans.: 5(x2 + y2 + z2) – 2x – 4y – 5z + 6 = 0 5(x2 + y2 + z2) – 2x – 4y – 5z + 1 = 0

23.

Find the equations of the sphere passing through the circle x2 + y2 + z2 – 5 = 0, 2x + 3y + z – 3 = 0 and touching the plane 3x + 4z – 15 = 0. Ans.: x2 + y2 + z2 + 4x + 6y – 2z – 11 = 0 5(x2 + y2 + z2) – 8x – 12y – 4z – 37 = 0

24.

Find the point of intersection of the line and the sphere x2 + y2 + z2 – 4x + 6y – 2z + 5 = 0. Ans.: (4, –1, 2), (0, –2, 3)

25.

Prove that the sum of the squares of the intercepts made by a given line on any three mutually perpendicular lines through a fixed point is constant.

x−a y−b z−c = = where l, m and n are the n l m direction cosines of a line, should touch the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0. Show that there are two spheres through the points (0, 0, 0), (2a, 0, 0), (0, 2b, 0) and (0, 0, 2c) which touch the above line and that the distance 1 2 between their centres is 2 [c 2 − ( a 2 + b 2 + c 2 )n2 ] 2 . n

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14-50

Analytical Geometry

Exercises 4 1.

Prove that the spheres x2 + y2 + z2 + 6y + 2z + 8 = 0 and x2 + y2 + z2 + 6x + 8y + 4z + 2 = 0 intersect orthogonally.

2.

Find the equation of the sphere which passes through the circle x2 + y2 + z2 – 2x + 3y – 4z + 6 = 0, 3x – 4y + 5z – 15 = 0 and which cuts orthogonally the sphere x2 + y2 + z2 + 2x + 4y – 6z + 11 = 0. Ans.: 5(x2 + y2 + z2) – 13x + 19y – 25z + 45 = 0

 3.



Find the equation of the sphere that passes through the circle x2 + y2 + z2 – 2x + 3y – 4z + 6 = 0, 3x – 4y + 5z – 15 = 0 and which cuts the sphere x2 + y2 + z2 + 2x + 4y + 6z + 11 = 0 orthogonally. Ans.: x2 + y2 + z2 + x – y + z – 9 = 0

4.

Prove that every sphere through the circle x2 + y2 – 2ax + r2 = 0, z = 0 cuts orthogonally every sphere through the circle x2 + z2 = r2, y = 0.

5.

Find the equation of the sphere which touches the plane 3x + 2y – z + 7 = 0 at the point (1, –2, 1) and cuts the sphere x2 + y2 + z2 – 4x + 6y + 4 = 0 orthogonally. Ans.: 3(x2 + y2 + z2) + 6x + 20y – 10z + 36 = 0

 6.

 7. 

Find the equation of the sphere that passes through the points (a, b, c) and (–2, 1, –4) and cuts orthogonally the two spheres x2 + y2 + z2 + x – 3y + 2 = 0 and (x2 + y2 + z2) + x + 3y + 4 = 0. Ans.: x2 + y2 + z2 + 2x – 2y + 4z – 3 = 0 Find the equation of the sphere which touches the plane 3x + 2y – z + 2 = 0 at the point P(1, –2, 1) and also cuts orthogonally. Ans.: x2 + y2 + z2 + 7x + 10y – 5z + 12 = 0

8.

If d is the distance between the centres of the two spheres of radii r1 and r2  r 2 + r22 − d 2  then prove that the angle between them is cos −1  1  . 2r r 

9.

Find the condition that the sphere a(x + y + z ) + 2lx + z – y + 2nz + p = 0 and b(x2 + y2 + z2)k2 may cut orthogonally. Ans.: ak2 = bp2

1 2

 10. 

2

2

2

Find the equation of the radical planes of the spheres x2 + y2 + z2 + 2x + 2y + 2z – 2 = 0, x2 + y2 + z2 + 4y = 0, x2 + y2 + z2 + 3x – 2y + 8z – 6 = 0. Ans.: x – y + z – 1 = 0, 3x – 6y + 8z – 6 = 0 x – 4y + 6z + 4 = 0

 11. 

Find the equation of the radical line of the spheres (x – 2) + y2 + z2 = 1, x2 + (y – 3)2 + z2 = 6 and (x + 2)2 + (y + 1)2 + (z – 2)2 = 6. x y z Ans.: = = 3 2 7

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2

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Sphere

12.  13.  14.  15.  16.



14-51

Find the equation of the radical line of the spheres x2 + y2 + z2 + 2x + 2y + 2z + 2 = 0, x2 + y2 + z2 + 4y = 0, x2 + y2 + z2 + 3x – 2y + 8z + 6 = 0. Ans.: x – y + z + 1 = 0, 3x – 6y + 8z + 6 = 0 Find the radical plane of the spheres x2 + y2 + z2 – 8x + 4y + 4z + 12 = 0, x2 + y2 + z2 – 6x + 3y + 3z + 9 = 0. Ans.: 2x – y – z – 3 = 0 Find the spheres coaxal with the spheres x2 + y2 + z2 + 2x + y + 3z – 8 = 0 and x2 + y2 + z2 – 5 = 0 and touching the plane 3x + 4y = 15. Ans.: 5(x2 + y2 + z2) – 8x – 4y – 12z – 13 = 0 Find the limiting points of the coaxal system defined by the spheres x2 + y2 + z2 + 3x – 3y + 6 = 0, x2 + y2 + z2 – 6y – 6z + 6 = 0. Ans.: (–1, 2, 1), (–2, 1, –1) Find the limiting points of the coaxal system determined by the two spheres whose equations are x2 + y2 + z2 – 8x + 2y – 2z + 32 = 0, x2 + y2 + z2 – 7x + z + 23 = 0. Ans.: (3, 1, –2), (5, –3, 4)

17.

Find the equations of the spheres whose limiting points are (–1, 2, 1) and (–2, 1, –1) and which touches the plane 2x + 3y + 6z + 7 = 0.

18.

Find the equation of the sphere which touches the plane 3x + 2y – z + 2 = 0 at the point (1, –2, 1) and also cut orthogonally the sphere x2 + y2 + z2 – 4x + 6y + 4 = 0. Ans.: x2 + y2 + z2 + 7x + 10y – 5z + 12 = 0

 19.  20.



Find the limiting points of the coaxal system two of whose members are x2 + y2 + z2 – 3x – 3y + 6 = 0, x2 + y2 + z2 – 4y – 6z + 6 = 0. Ans.: (2, –3, 4) and (–2, 3, –4) The point (–1, 2, 1) is a limiting point of a coaxal system of spheres of which the sphere x2 + y2 + z2 + 3x – 2y + 6 = 0 is a member. Find the coordinates of the other limiting point. Ans.: (–2, 1, –1)

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Chapter

15

Cone

15.1  Definition of Cone A cone is a surface generated by a straight line which passes through a fixed point and intersects a fixed curve or touches a given curve. The fixed point is called the vertex of the cone and the fixed curve is called a guiding curve of the cone. The straight line is called a generator.

15.2  Equation of a cone with a given vertex and a given guiding curve Let (a, b, g  ) be the given vertex and ax2 + 2hxy + by2 + 2gx + 2fy + c = 0, z = 0 be the guiding curve. The equations of any line passing through the point (a, b, g  ) are x −a y − b z −g = = l m n



(15.1)

When this line meets the plane at z = 0 we get,

x −a y − b −g = = l m n lg mg ∴x = a − , y = b − , z=0 n n

This point lies on the given curve ax2 + 2hxy + by2 + 2gx + 2fy + c = 0, z = 0. 2

2

lg  lg   mg  mg     ∴a  a −  + 2 h  a −   b − + b b −   +    n n n  n 

lg   2g a −  + 2 f  n

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mg    b −  +c = 0 n 



(15.2)

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15-2

Analytical Geometry

Eliminating l, m, n from (15.1) and (15.2) we get the equation of the cone. From (15.1) 

l x −a m y−b = and = n z −g n z −g 2



   x −a  x −a   y−b  y−b  g a a − g  + 2h  a − g  b − g  + b b −   z −g   z −g   z −g  z − g   x −a  +2 g  a − g +2f z − g  

2

 y−b   b − z − g g  + c = 0

Multiplying throughout by (z - g  )2, we get a(a z - g x)2 + 2h(a z - g x) (b z - g y) + b(b z - g y)2 + 2g(a z - g x)(z - g ) + zf (b z - g y)(z - g ) + c(z - g )2 = 0. This is the required equation of the cone.

Example 15.2.1 Find the equation of the cone with its vertex at (1, 1, 1) and which passes through the curve x2 + y2 = 4, z = 2.

Solution Let V be the vertex of the cone and P be any point on the surface of the cone. Let the equations of the generator VP be x −1 y −1 z −1 = = l m n



(15.3)

This line intersects the plane z = 2.

x −1 y −1 1 = = m n l

∴

l m ∴ x = 1+ , y = 1+ n n



This point lies on the curve x2 + y2 = 4. 2

or

2

l   m  ∴ 1 +  + 1 +  = 4  n  n (l + n)2 + (m + n)2 = 4n2

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(15.4)

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Cone

15-3

Eliminating l, m, n from (15.3) and (15.4) we get 2

2

y − 1  x − 1  1 +  + 1 +  =4 z − 1  z − 1

or

 (z - 1 + x - 1)2 + (z - 1 + y - 1)2 = 4(z - 1)2 (x + z - 2)2 + ( y + z - 2)2 = 4(z - 1)2



This is required equation of the cone.

Example 15.2.2 Find the equation of the cone whose vertex is (a, b, c) and whose base is the curve a x2 + b y2 = 1, z = 0.

Solution The vertex of the cone is V(a, b, c). The guiding curve is a x2 + b y2 = 1, z = 0



(15.5)

Let l, m, n be the direction cosines of the generator VP. Then the equations of VP are x−a y−b z−c = = n l m



(15.6)

When this line meets z = 0 we have x − a y − b −c = = l m n cl cm ∴x = a− , y = b− n n 2 2 The point lies on the curve a x + b y = 1.

2



2

cl  cm    ∴a  a −  + b  b −  = 1   n n

(15.7)

We have to eliminate l, m, n from (15.6) and (15.7) 2



2

x − a y − b   a a −c  + b  b − c  =1  z−c z −c

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15-4

or

Analytical Geometry

a [a(z - c) - c(x - a)]2 + b  [b(z - c) - c(y - b)]2 = (z - c)2

(i.e.)  a c2x2 + b c2y2 + (a a2 + b b2 - 1)2z2 - 2bcb yz - 2aca  xz + 2zc - c2 = 0 This is the equation of the required cone.

15.3  Equation of a cone with its vertex at the origin To show that the equation of a cone with its vertex at the origin is homogeneous, let

f(x, y, z) = 0

(15.8)

be the equation of a cone with its vertex at the origin. Let P(a, b, g ) be any point on the surface. Then, OP is a generator of the cone. Since (a, b, g ) lies on the cone

f(x, y, z) = 0

(15.9)

The equations of OP are

x y z = = = t a b g

(15.10)

Any point on this line is (ta, tb, tg  ). The point lies on the cone f(x, y, z) = 0.

\  f(ta, tb, tg  ) = 0

(15.11)

From equations (15.8) and (15.11), we observe that the equation f(x, y, z) = 0 is homogeneous. Conversely, every homogeneous equation in (x, y, z) represents a cone with its vertex at the origin. Let f (x, y, z) = 0 be a homogeneous equation in x, y, z. Since f (x, y, z) = 0 is a homogeneous equation, f(x, y, z) = 0 for any real number. In particular f (0, 0, 0) = 0. Therefore, the origin lies on the locus of the equation (15.8). As f(tx, ty, tz) = 0, any point on the line through the origin lies on the equation (15.8). In other words, the locus of (15.8) is a surface generated by the line through the origin. Hence equation represents a cone with its vertex at the origin.

Note 15.3.1: If f (x, y, z) can be expressed as the product of n linear factors then f (x, y, z) = 0 represents n planes through the origin.

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Cone

15-5

Note 15.3.2: If f (x, y, z) = 0 is a homogeneous equation of second degree in x, y and z then f (x, y, z) = 0 is a quadric cone. If it can be factored into two linear factors then it represents a pair of planes through the origin; then we regard equation f (x, y, z) = 0 as degenerate cone, the vertex being any point on the line of intersection of the two planes.

x y z = = is a generator of the cone f (x, y, z) = 0 with l m n its vertex at the origin if and only if f (l, m, n) = 0.

Generators: The line

x y z = = = r be a generator of the cone f (x, y, z) = 0 then the point l m n (lr, mr, nr) lies on the cone. Taking r = 1, the point (l, m, n) lies on the cone f (x, y, z) = 0. Let



\  f (l, m, n) = 0

Converse: Let f (x, y, z) = 0 be the equation of the cone with its vertex at the origin and

x y z = = = r be a line through the origin such that f (l, m, n) = 0. l m n

Since the vertex is at the origin, f (x, y, z) = 0 is a homogeneous equation in x, y and z. x y z Now we will prove that = = is a generator of the cone. l m n Any point on the generator is (lr, mr, nr). Since f (x, y, z) = 0 and f (l, m, n) = 0, it follows that f (lr, mr, nr) = 0.

x y z ∴ = = is a generator of the cone f (x, y, z) = 0. l m n

Example 15.3.1 Find the equation of the cone with its vertex at the origin and which passes through the curve ax2 + by2 + cz2 - 1 = 0 = a x2 + by2 - 2z.

Solution Let the equation of the generator be

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x y z = = = r l m n

(15.12)

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15-6

Analytical Geometry

Any point on this line is (lr, mr, nr). This point lies on the curve  ax2 + by2 + cz2 = 1

a x2 + b y2 - 2z = 0



r2(al 2 + bm2 + cn2) = 1

(15.13)



r(a rl 2 + brm2 - 2n) = 0

(15.14)

r=

From (15.14),

2n a l 2 + b m2

Substituting this in (15.13) we get

4 n2 ( al 2 + bm2 + cn2 ) = 1 (a l + b m2 )2 2

(i.e.)  4n2 (al 2 + bm2 + cn2) = (a l 2 + b m2)2

As l, m, n are proportional to x, y, z the equation of the cone is 4z2(ax2 + by2 + cz2) = (a x2 + by2)2.

Example 15.3.2 Find the equation of the cone whose vertex is at the origin and the guiding x2 y2 + + z 2 = 1, x + y + z = 1. curve is 4 9

Solution Since the vertex of the cone is the origin its equation must be a homogeneous equation of second degree. The equations of the guiding curve are

x2 y2 + + z 2 = 1 4 9

(15.15)



x + y + z = 1

(15.16)

Homogenizing the equation (15.15) with the help of (15.16) we get the equation of the required cone. Hence the equation of the cone is x2 y2 + + z 2 = ( x + y + z )2. 4 9

(i.e.)  27x2 + 32y2 + 7z2 (xy + yz + zx) = 0

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15-7

Cone

Example 15.3.3 x y z + + = 1 meets the coordinate axes at A, B and C. Prove that a b c the equation of the cone generated by lines drawn from O to meet the  b c  c a  a b circle ABC is yz  +  + zx  +  + xy  +  = 0.  c b  a c  b a

The plane

Solution The points A, B, C are (a, 0, 0), (0, b, 0) and (0, 0, c), respectively. The equation of the sphere OABC is x2 + y2 + z2 - ax - by - cz = 0. The equations of the circle ABC are  x2 + y2 + z2 - ax - by - cz = 0

(15.17)

x y z + + = 1 a b c

(15.18)



Homogenizing equation (15.17) with the help of (15.18) we get the equation of the required cone. (i.e.)

 x y z x 2 + y 2 + z 2 − ( ax + by + cz )  + +  = 0  a b c

 c a  a b  b c (i.e.)  +  yz +  +  zx +  +  xy = 0  a c  b a  c b

15.4  Condition for the general equation of the second degree to represent a cone Let the general equation of the second degree be    ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy + 2ux + 2vy + 2wz + d = 0 (15.19) Let (x1, y1, z1) be the vertex of the cone. Shift the origin to the point (x1, y1, z1). Then

x = X + x1



y = Y + y1



z = Z + z1

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15-8

Analytical Geometry

Then the equation (15.19) becomes a (X + x1)2 + b(Y + y1)2 + c(Z + z1)2 + 2f (Y + y1) (Z + z1) + 2g(Z + z1) (X + x1) + 2h(X + x1) (Y + y1) + 2u(X + x1) + 2v(Y + y1) + 2w(Z + z1) + d = 0 (i.e.) aX 2 + bY 2 + cZ 2 + 2 fyZ + 2 gZX + 2hXY + 2( ax1 + hy1 + gz1 + u ) X

+2( hx1 + by1 + fz1 + v )Y + 2( gx1 + fy1 + cz1 + w ) Z  + 2ux1 + 2vy1 + 2wz1 + ( ax12 + by12 + cz12 + 2 fy1 z1 + 2 gz1 x1 + 2hx1 y1 + d) = 0

Since this equation has to be a homogeneous equation in X, Y and Z. Coefficient of X = 0 Coefficient of Y = 0 Coefficient of Z = 0 and constant term = 0.

\  ax1 + hy1 + gz1 + u = 0

(15.20)



hx1 + by1 + fz1 + v = 0

(15.21)



gx1 + fy1 + cz1 + w = 0

(15.22)

ax12 + by12 + cz12 + 2 fy1 z1 + 2 gz1 x1 + 2hx1 y1

+2ux1 + 2vy1 + 2wz1 + d = 0



(15.23)

(i.e.) x1(ax1 + hy1 + gz1 + u) + y1 (hx1 + by1 + fz1 + v) + z1 (gx1 + fy1 + cz1 + w) + ux1 + vy1 + wz1 + d = 0

= 0 + 0 + 0 + ux1 + vy1 + wz1 + d = 0



\  ux1 + vy1 + wz1 + d = 0

(15.24)

Eliminating x, y, z from (15.21), (15.22), (15.23) and (15.24), we get,



a h g u

h b f v

g u f v =0 c w w d

This is the required condition.

Note 15.4.1: If the given equation of the second degree is f (x, y, z) = 0 then make it homogeneous by introducing the variable t where t = 1.

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Cone

15-9

Then ∂F ∂F ∂F ∂F = 0, = 0, = 0, = 0 where t = 1 ∂x ∂y ∂z ∂t



Solving any three of these four equations, we get the vertex of the cone. Test whether these values of x, y, z satisfy the fourth equation.

Example 15.4.1 Find the equation of the cone of the second degree which passes through the axes.

Solution The cone passes through the axes. Therefore, the vertex of the cone is the origin. The equations of the cone is a homogeneous equation of second degree in x, y and z. (i.e.)  ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0 (15.25) Given that x-axis is a generator. Then y = 0, z = 0 must satisfy the equation (15.25)

\  a = 0 Since y-axis is a generator b = 0. Since z-axis is a generator  c = 0. Hence the equation of the cone is fyz + gzx + hxy = 0.

Example 15.4.2 Show that the lines through the point (a, b, g  ) whose direction cosines satisfy the relation al2 + bm2 + cn2 = 0, generates the cone a(x - a)2 + b(y - b)2 + c(z - g  )2 = 0.

Solution The equations of any line through (a, b, g  ) are

x y z = = l m n

(15.26)

al 2 + bm2 + cn2 = 0

(15.27)

where

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15-10

Analytical Geometry

Eliminating l, m, n from (15.26) and (15.27) we get, a(x - a )2 + b(y - b  )2 + c(z - g  )2 = 0



Example 15.4.3 Find the equation to the quadric cone which passes through the three cox y z ordinate axes and the three mutually perpendicular lines = = , 1 −2 3 x y z x y z = = and = = . 1 −1 −1 5 4 1

Solution We have seen that the equation of the cone passing through the axes is fyz + gzx + hxy = 0



This cone passes through line

(15.28)

x y z = = . 1 −2 3

\  f (-6) + g(3) + h(-2) = 0

or

6f - 3g + 2h = 0

Since the cone also passes through the line

(15.29)

x y z = = we have 1 −1 −1

f - g - h = 0

(15.30)

From (15.29) and (15.30) we get

f g h = = 5 8 −3

(15.31)

From (15.28) and (15.31) we get 5yz + 8zx - 3xy = 0. Since the cone passes through the line

x y z = = , 5 4 1

we get 4f + 5g + 20h = 0 and 4(5) + 5(8) + 20(-3) = 0 is also true. Therefore, the equation of the required cone is 5yz + 8zx - 3xy = 0.

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Cone

15-11

Example 15.4.4 Prove that the equation 2x2 + 2y2 + 7z2 - 10yz - 10zx + 2x + 2y + 26z - 17 = 0 represents a cone whose vertex is (2, 2, 1).

Solution Let F(x, y, z, t) = 2x2 + 2y2 + 7z2 - 10yz - 10zx - 2xt + 2yt + 26zt - 17t2 = 0



∂F = 4 x − 10 z + 2t ∂x ∂F = 4 y − 10 z + 2t ∂y



∂F = 14 z − 10 y − 10 x + 26t ∂z ∂F = 2 x + 2 y + 26 z − 34t ∂t

at t = 1,

∂F ∂F ∂F ∂F = 0, = 0, = 0 and =0 ∂x ∂y ∂z ∂t

give the equations

2x - 5z + 1 = 0



2y - 5z + 1 = 0



5x + 8y - 7z - 13 = 0



x + y + 12z - 17 = 0

Solving the first three equations we get x = 2, y = 2, z = 1. These values also satisfy the fourth equation. Therefore, the given equation represents a cone with its vertex at the point (2, 2, 1).

Exercises 1 1.



Find the equation of the cone whose vertex is at the origin and which passes through the curve of intersection of the plane lx + my + nz = p and the surface, ax2 + by2 + cz2 = 1. Ans.: p 2 ( ax 2 + by 2 + cz 2 ) = (lx + my + nz )2

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15-12

Analytical Geometry



Find the equation of the cone whose vertex is (a, b, g  ) and whose guiding curve is the parabola y2 = 4ax, z = 0. Ans.: ( ry − b z )2 = 4 a( z − g )(a z − rx )

3.

Prove that the line

2.

x y z = = where 2l 2 + 3m2 - 5n2 = 0 is a generator of the l m n

cone 2x2 + 3y2 - 5z2 = 0. 4. 

Find the equation of the cone whose vertex is at the point (1, 1, 0) and whose guiding curve is x2 + z2 = 4, y = 0. Ans.: x 2 − 3 y 2 + z 2 − 2 xy + 8 y − 4 = 0

5.

Find the equation of the cone whose vertex is the point (0, 0, 1) and whose x2 y2 + = 1, z = 3. Also obtain section of the cone 15 9 by the plane y = 0 and identify its type. Ans.: 36 x 2 + 100 y 2 − 225 z 2 + 450 z − 225 = 0

guiding curve is the ellipse 

5 x = ± ( z − 1); pair of straight lines 2

 6.

Find the equations of the cones with vertex at the origin and passing through the curves of intersection given by the equations:



(i) ax2 + by2 = 22, lx + my + nz = 1



(ii)



(iii)



(iv) x 2 + y 2 + z 2 + x − 2 y + 3 z = 4



(v) x 2 + y 2 + z 2 + 2 x − 3 y + 4 = 5

x2 y2 z2 x2 y2 + 2 + 2 = 1, 2 + 2 = 2 z 2 a b a b c x 2 + y 2 + z 2 + x − 2 y + 3z − 4 = 0 x+ y+z=2

Ans.: (i)  p( ax 2 + by 2 ) = 2 z (lx + my + nz )



 x2

y2

z2 

 x2

y2 

2

2 (ii)  4 z  2 + 2 + 2  =  a 2 + b 2  b c   a 

 

(iii)  2 x 2 + 3 y 2 + 4 z 2 − xy − 3 yz − 2 zx = 0



(iv)  8 x 2 + y 2 − 5 xy − 3 yz − 4 zx = 0

7.

The plane x + y + z = 1 meets the coordinate axes in A, B and C. Prove that the equation to the cone generated by the lines through O, to meet the circle ABC is yz + zx + xy = 0.

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Cone

15-13

x y z + + = 0 and meets the a b c axes in A, B and C. Prove that, the circle ABC lies on the cone  b c  c a  a b yz  +  + zx  +  + xy  +  = 0 .  c a  a c  b a

8.

A variable plane is parallel to the plane

9.



i) Find the equation of the quadric cone which passes through the three coordinate axes and three mutually perpendicular lines x y z x y z x y z = = , = = , = = . 2 1 −1 1 3 5 8 −11 5 Ans.: 16 yz − 33 zx − 25 xy = 0



ii) Prove that the equation of the cone whose vertex is (0, 0, 0) and the base  xk yk  curve z = k , f ( x, y ) = 0 is f  ,  = 0, where f (x, y) = ax2 + 2hxy +  z z  by2 + 2gx + 2fy + c = 0.

10.



Find the equation to the cone whose vertex is the origin and the base circle x = a, y2 + z2 = b2 and show that the section of the cone by a plane parallel to the xy-plane is hyperbola. Ans.: a 2 ( y 2 + z 2 ) = b 2 x 2

11.

Planes through OX and OY include an angle a. Show that the line of intersection lies on the cone z2(x2 + y2 + z2) = x2y2 tan2 a .

12.

Prove that a cone of second degree can be found to pass through two sets of rectangular axes through the same origin.

13.

Prove that the equation x2 - 2y2 + 3z2 + 5yz - 6zx - 4xy + 8x - 19y - 2z - 20 = 0 represents a cone with its vertex at (1, -2, 3).

14.

Prove that the equation 2y2 - 8yz - 4zx - 8xy + 6x - 4y - 2z + 5 = 0 represents  −7 1 5  a cone whose vertex is  , ,  . 6 3 6

15.

Prove that the equation ax2 + by2 + cz2 + 2ux + 2vy + 2wz + d = 0 represents a u2 v2 w 2 + + = d. cone if a b c

15.5  Right circular cone A right circular cone is a surface generated by a straight line which passes through a fixed point, and makes a constant angle with a fixed straight line through the fixed point. The fixed point is called the vertex of the cone and

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15-14

Analytical Geometry

the constant angle is called the semi-vertical angle and fixed straight line is called the axis of the cone. The section of right circular cone by any plane perpendicular to its axis is a circle.

15.5.1  Equation of a Right Circular Cone with Vertex V (`, a, f ), Axis VL with Direction Ratios l, m, n and Semi-vertical Angle p Let P(x, y, z) be any point on the surface of the cone. Then direction ratios of VP are x - a, y - b, z - g . V(a, b, g )

P(x, y, z)

A

L

B

The direction ratios of the perpendicular VL are l, m, n. Now,

cos q =

( x − a )l + ( y − b ) m + ( z − g ) n ( x − a ) 2 + ( y − b ) 2 + ( z − g ) 2 l 2 + m 2 + n2

(i.e.) [(x - a)l + (y - b )m + (z - g )n]2 = [(x - a)2 + (y - b )2 + (z - g )2] (l 2 + m2 + n2)cos2q (15.32) This is the required equation of the cone.

Note 15.5.1.1: (i) If the vertex is at the origin then the equation of the cone becomes (lx + my + nz)2 = [(x2 + y2 + z2)(l2 + m2 + n2)]cos2q. (ii) If l, m, n are the direction cosines of the line then

(lx + my + nz)2 = (x2 + y2 + z2)cos2q

(15.33)

(iii) If axis of cone is the z-axis then the equation (15.33) becomes

z2 = (x2 + y2 + z2)cos2q z2 sec2q = x2 + y2 + z2

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Cone

15-15

x2 + y2 = z2(sec2q - 1) x2 + y2 = z2 tanq

or

15.5.2  Enveloping Cone It has been seen in the two-dimensional analytical geometry that two tangents can be drawn from a given point to a conic. In analogy with that an infinite number of tangent lines can be drawn from a given point to a conicoid, in particular to a sphere. All such tangent lines generate a cone with the given point as vertex. Such a cone is called an enveloping cone.

Definition 15.5.2.1:  The locus of tangent lines drawn from a given point to a given surface is called an enveloping cone of the surface. The given point is called the vertex of the cone.

Equation of enveloping cone: Let us find the equation of the enveloping cone of the sphere x2 + y2 + z2 = a2 with the vertex at (x, y, z). P

V

Let P(x, y, z) be any point on the tangent drawn from V(x1, y1, z1) to the given sphere. Let Q be the point that divides PQ in the ratio 1:l. Then the coordi l x + x1 l y + y1 l z + z1  , , . nates of Q are   1+ l 1+ l 1 + l  If this point lies on the sphere then, 2



2

2

 l x + x1   l y + y1   l z + z1  2  1 + l  +  1 + l  +  1 + l  = a (i.e.)  ( l x + x1 )2 + ( l y + y1 )2 + ( l z + z1 )2 = a2 (1 + l )2 or (x 2 + y 2 + z 2 − a 2 )l 2 + 2( xx1 + yy1 + zz1 − a2 )l + ( x12 + y12 + z12 − a2 ) = 0

M15_ANALYTICALGEOMETRY_ISBN_CH15.indd 15



(15.34)

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15-16

Analytical Geometry

This is a quadratic equation in l. There are two values of l indicating that there are two points on VP which divides PQ in the ratio 1:l and lie on the sphere. If PQ is a tangent to the sphere then these two points coincide and the point is the point of contact. Therefore, the two values of l of equation (15.34) must be equal and hence the discriminant must be zero. \(xx1 + yy1 + zz1 - a2)2 = (x2 + y2 + z2 - a2) (x12 + y12 + z12 - a2). This is the equation of the enveloping cone.

Note 15.5.2.2: The equation of the enveloping cone can be expressed in the form T 2 = SS1 where

T = xx1 + yy1 + zz1 − a 2 S = x 2 + y 2 + z 2 − a2 S1 = x12 + y12 + z12 − a2

Example 15.5.1 Find the equation of the right circular cone whose vertex is at the origin, x y z whose axis is the line = = and which has a vertical angle of 60°. 1 2 3

Solution The axis of the cone is

x y z = = . 1 2 3 O

30° L

P

A

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Cone

15-17

Therefore, the direction ratios of the axis of the cone are 1, 2, 3. 1 2 3 The direction cosines are , , . 14 14 14 Let P(x, y, z) be any point on the surface of the cone. Let PL be perpendicular to OA. ∠POL = 30° OL = cos 30° or 2OL = 3OP OP Also, OP2 = x2 + y2 + z2

OL = Projection of OP on OA =

x 14

2

+ y× ∴

+z×

3

14 14 2 ( x + 2 y + 3z ) 14

=

x + 2 y + 3z 14

= 3 x2 + y2 + z2

4 ( x + 2 y + 3 z ) = 42( x 2 + y 2 + z 2 ) 2

or

19 x 2 + 13 y 2 + 3 z 2 − 8 xy − 24 yz − 12 zx = 0

Example 15.5.2 Find the equation of the right circular cone with its vertex at the origin, axis along the z-axis and semi-vertical angle a.

Solution The direction cosines of the axis of the cone are 0, 0, 1. Let P(x, y, z) be any point on the cone. O a

Z P(x, y, z)

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15-18

Analytical Geometry

z

or ( x 2 + y 2 + z 2 ) cos 2 a = z 2 or x 2 + y 2 = z 2 tan 2 a x + y2 + z2 This is the required equation of the cone. Then, cosa =

2

Example 15.5.3 Find the semi-vertical angle and the equation of the right circular cone having its vertex at origin and passing through the circle y2 + z2 = b2, x = a.

Solution The guiding circle of the right circular cone is y2 + z2 = b2, x = a. Therefore, the axis of the cone is along x-axis. a If q  is the semi-vertical angle, then cosq = . a2 + b2 Let P(x, y, z) be any point on the surface of the cone. The direction ratios of OP are x, y, z. O q

a

a2 + b2 P(x, y, z)

M

b

The direction cosines of the x-axis are 1, 0, 0.

or

∴ cosq = a a +b

M15_ANALYTICALGEOMETRY_ISBN_CH15.indd 18

2

2

=

x x + y2 + z2 2

x x + y2 + z2 2

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Cone



15-19

a2(x2 + y2 + z2) = x2(a2 + b2) (i.e.)  b x - a2(y2 + z2) = 0 2 2

which is the required equation of the cone.

Example 15.5.4 A right circular cone has its vertex at (2, -3, 5). Its axis passes through A(3, -2, 6) and its semi-vertical angle is 30°. Find its equation.

Solution The axis is the line joining the points (2, -3, 5) and (3, -2, 6). x −2 y+3 z −5 = = . Therefore, its equations are 1 1 1 The direction ratios of the axes are 1, 1, 1. 1 1 1 The direction cosines are , , . 3 3 3 Let P(x, y, z) be any point on the cone. A

°

30

L



P

∠PAL = 30°



AL 3 = cos 30° = AP 2



AL =



AP = ( x − 2)2 + ( y + 3)2 + ( z − 5)2

x−2

M15_ANALYTICALGEOMETRY_ISBN_CH15.indd 19

3

+

y+3 3

+

z −5 3

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15-20

Analytical Geometry



∴

( x + y + z − 4)

1

3 ( x − 2)2 + ( y + 3)2 + ( z − 5)2

=

3 2

Squaring, cross multiplying and simplifying we get, 5x2 + 5y2 + 5z2 - 8xy 8yz - 8zx - 4x + 86y - 58z + 278 = 0.

Example 15.5.5 A right circular cone has three mutually perpendicular generators. Prove that the semi-vertical angle of the cone is tan-1 2 .

Solution The equation of the right circular cone with vertex at the origin, semi-vertical angle a and axis along z-axis is given by x2 + y2 = z2 tan2a. This cone will have three mutually perpendicular generators if coefficient of x2 + coefficient of y2 + coefficient of z2 = 0.

\  1 + 1 - tan2q  = 0

or

tanq  = 2



\  q  = tan-1( 2)

Example 15.5.6 The axis of a right cone vertex O, makes equal angles with the coordinate axes and the cone passes through the line drawn from O with direction ­cosines proportional to (1, -2, 2). Find the equation to the cone.

Solution Let the axis of the cone make an angle b with the axes. Then the direction cosines of the axes are cosb, cosb, cosb. (i.e.) 1, 1, 1. Let a be the semi-vertical angle of the axis of the cone. The direction ratios of one of the generators are 1, -2, 2.

∴cosa

1− 2 + 2 1+1+1 1+ 4 + 4

=

1 3 3

Let P(x, y, z) be any point on the cone. Then the direction ratios of OP are x, y, z. The direction ratios of the axis are 1, 1, 1.

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Cone

∴ cosa =



1



3 3

=

15-21

x+ y+z 3 x2 + y2 + z2 x+ y+z 3 x2 + y2 + z2

Squaring and cross multiplying we get, 9(x + y + z)2 = x2 + y2 + z2 or 4x2 + 4y 2 + 4z2 + 9xy + 9yz + 9zx = 0

Example 15.5.7 Prove that the line drawn from the origin so as to touch the sphere x2 + y2 + z2 + 2ux + 2vy + 2wz + d = 0 lie on the cone d(x2 + y2 + z2) = (ux + vy + wz)2.

Solution The lines drawn from the origin to touch the sphere generates the enveloping cone. The equation of the enveloping cone of the given sphere is T 2 = SS1.

(i.e.)  (ux + vy + wz + d)2 = (x2 + y2 + z2 + 2ux + 2vy + 2wz + d )d (i.e.)  d(x2 + y2 + z2) = (ux + vy + wz)2

Example 15.5.8 Show that the plane z = 0 cuts the enveloping cone of the sphere x2 + y2 + z2 = 11 which has its vertex at (2, 4, 1) in a rectangular hyperbola.

Solution The equation of the enveloping cone with its vertex at (2, 4, 1) is T 2 = SS1. S = x2 + y2 + z2 - 11



S12 = 4 + 16 + 1 - 11 = 10



T = 2x + 4y + z - 11



\ (2x + 4y + z - 11)2 = 10(x2 + y2 + z2 - 11)

The section of this cone by the plane z = 0 is (2x + 4y - 11)2 = 10(x2 + y2 - 11). Coefficient of x2 + coefficient of y2 = 6 - 6 = 0 Hence, the plane z = 0 cuts the enveloping cone in a rectangular hyperbola.

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15-22

Analytical Geometry

Exercises 2 1.



Find the equation of the right circular cone whose vertex is the line x y z = = and which has a vertical angle of 60°. 1 2 3 Ans.: 19 x 2 + 13 y 2 + 3 z 2 − 8 xy − 24 yz − 12 zx = 0

2.

If (x, y, z) is any point on the cone whose vertex is (1, 0, 2) and semi-vertical angle is 30° and the equation to the axis is

x −1 y z − 2 = = , show that the −2 1 2

equation of the cone is 27[( x − 1)2 + y 2 + ( z − 2)2 ] = 4( x + 2 y − 2 z + 3)2. 3.  4.

 5.

 6.  7.

 8.  9. 

Find the equation to the right circular cone of semi-vertical angle 30°, whose vertex is (1, 2, 3) and whose axis is parallel to the line x = y = z. Ans.: 5( x 2 + y 2 + z 2 ) − 8( yz + zx + xy ) + 30 x + 12 y − 6 z − 18 = 0 Find the equation to the right circular cone whose vertex is (3, 2, 1), semix − 3 y − 2 z −1 vertical angle is 30° and axis is the line = = . 4 1 3 Ans.: 7 x 2 + 37 y 2 + 21z 2 − 16 xy − 12 yz − 48 zx + 38 x − 88 y + 126 z − 32 = 0 Find the equation of the right circular cone with vertex at (1, -2, -1), x −1 y +1 z +1 semi-vertical angle 60° and axis = = . 3 −4 5 Ans.: 5[(5 x + 4 y + 14)2 + (3 z − 5 x + 8)2 + ( 4 x + 3 y + 2)2 ] = 75[( x − 1)2 + ( y + 2)2 + ( z + 1)2 ] Find the equation of the right circular cone which passes through the three lines drawn from the origin with direction ratios (1, 2, 2) (2, 1, -2) (2, -2, 1). Ans.: 8 x 2 − 5 y 2 − 4 z 2 + yz + 5 zx + 5 xy = 0 Lines are drawn through the origin with direction cosines proportional to (1, 2, 2), (2, 3, 6), (3, 4,12). Find the equation of the right circular cone through them. Also find the semi-vertical angle of the cone.  1  Ans.: xy − yz + zx = 0;cos −1    3 Find the equation of the cone generated when the straight line 2y + 3z = 6, x = 0 revolves about the z-axis. Ans.: 4x2 + 4y2 - 9z2 + 36z - 36 = 0 Find the equation to the right circular cone which has the three coordinate axes as generators. Ans.: xy + yz + zx = 0

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15-23

Cone

10. 

Find the equation of the right circular cone with its vertex at the point (0, 0, 0), its axis along the y-axis and semi-vertical angle q. Ans.: x2 + z2 = y2 tan2q

11.

If a is the semi-vertical angle of the right circular cone which passes through

(

the lines ox, oy, x = y = z, show that cos a = 9 − 4 3

)

−

1 2

.

12.

Prove that x2 + y2 + z2 - 2x + 4y + 6z + 6 = 0 represents a right circular cone whose vertex is the point (1, 2, -3), whose axis is parallel to oy and whose semi-vertical angle is 45°.

13.

Prove that the semi-vertical angle of a right circular cone which has three mutually perpendicular tangent planes is cot −1 2 .

14.

Find the enveloping cone of the sphere x2 + y2 + z2 - 2x - 2y = 2 with its vertex at (1, 1, 1). Ans.: 3x2 - y2 + 4zx - 10x + 2y - 4z + 6 = 0

 15.

Find the enveloping cone of the sphere x2 + y2 + z2 = 11 which has its vertex at (2, 4, 1) and show that the plane z = 0 cuts the enveloping cone in a rectangular hyperbola.

15.6  Tangent plane Tangent plane from the point (x1, y1, z1) to the cone ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0. The equations of any line through the point (x1, y1, z1) are

x − x1 y − y1 z − z1 = = = r l m n

(15.35)

Any point on the line is (x1 + lr, y1 + mr, z1 + nr). If this point lies on the given cone then a(x1 + lr)2 + b(y1 + mr)2 + c(z1 + nr)2 + 2f(y1 + mr)(z1 + nr) + 2g(z1 + nr)(x1 + lr) + 2h(x1 + lr)(y1 + mr) = 0 (i.e.) ( al 2 + bm2 + cn2 + 2 fmn + 2 gnl + 2hlm)r 2 +   2[( ax1 + hy1 + gz1 )l + ( hx1 + by1 + fz1 )m + ( gx1 + fy1 + cz1 )n]r + ( ax12 + by12 + cz12 + 2 fy1 z1 + 2 gz1 x1 + 2hx1 y1 ) = 0

(15.36)

This equation is a quadratic in r. Since (x1, y1, z1) is a point on the cone

( ax12 + by12 + cz12 + 2 fy1 z1 + 2 gz1 x1 + 2hx1 y1 ) = 0

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(15.37)

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15-24

Analytical Geometry

Therefore, one root of the equation (15.36) is zero. If line (15.35) is a tangent to the curve, then both the roots are equal and hence the other root must be zero. The condition for this is the coefficient of r = 0. (i.e.)  ( ax1 + hy1 + gz1 )l + ( hx1 + by1 + fz1 )m + ( gx1 + fy1 + cz1 )n = 0 (15.38) Hence this is the condition for the line (15.35) to be a tangent to the curve at the point (x1, y1, z1). Since equation (15.38) can be satisfied for infinitely many values of l, m, n there are infinitely many tangent lines at any point of the cone. The locus of all such tangent lines is obtained by eliminating l, m, n from (15.35) and (15.38). ∴ ( ax1 + hy1 + gz1 )( x − x1 ) + ( hx1 + by1 + fz1 )( y − y1 ) +( gx1 + fy1 + cz1 )( z − z1 ) = 0



(i.e.)( ax1 + hy1 + gz1 ) x + ( hx1 + by1 + fz1 ) y + ( gx1 + fy1 + cz1 ) z = ax12 + by12 + cz12 + 2 fy1 z1 + 2 gz1 x1 + 2hx1 y1 = 0 by (15.37)

Therefore, the equation of the tangent plane at (x1, y1, z1) is ( ax1 + hy1 + gz1 ) x + ( hx1 + by1 + fz1 ) y + ( gx1 + fy1 + cz1 ) z = 0 .

Note 15.6.1: (0, 0, 0) satisfies the above equation and hence the tangent plane at any point of a cone passes through the vertex. The tangent plane at any point of a cone touches the cone along the generator through P.

Proof:

Let the equation of the cone be ax 2 + by 2 + cz 2 + 2 fyz + 2 gzx + 2hxy = 0 . Let P(x1, y1, z1) be any point on the cone. The equation of the tangent plane at P is ( ax1 + hy1 + gz1 ) x + ( hx1 + by1 + fz1 ) y + ( gx1 + fy1 + cz1 ) z = 0. x y z = = = r. x1 y1 z1 Any point on this line is (rx1, ry1, rz1). The equation of the tangent plane at (rx1, ry1, rz1) is (arx1 + hry1 + grz1)x + (hrx1 + bry1 + frz1)y + (grx1 + fry1 + crz1) z = 0. The equations of the generator through P are

Dividing by r,

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Cone

15-25

(ax1 + hy1 + gz1)x + (hx1 + by1 + fz1)y + (gx1 + fy1 + cz1)z = 0 which is also the equation of the tangent plane at (x1, y1, z1). Therefore, the tangent plane at P touches the cone along the generator through P.

15.6.1  Condition for the Tangency of a Plane and a Cone Let the equation of the cone be

ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0

(15.39)

Let the equation of the plane be lx + my + nz = 0



(15.40)

Let the plane (15.40) touch the cone at (x1, y1, z1). The equation of the tangent plane at (x1, y1, z1) is

(ax1 + hy1 + gz1)x + (hx1 + by1 + fz1)y + (gx1 + fy1 + cz1)z = 0 (15.41)

If the plane (15.40) touches the cone (15.39) then equations (15.40) and (15.41) are identical. Therefore identifying (15.40) and (15.41) we get,

ax1 + hy1 + gz1 hx1 + by1 + fz1 gx1 + fy1 + cz1 = = = r1 l m n

(15.42)



\ ax1 + hy1 + gz1 - r1l = 0

(15.43)



hx1 + by1 + fz1 - r1m = 0

(15.44)



gx1 + fy1 + cz1 - r1n = 0

(15.45)

Also as (x1, y1, z1) lies on the plane lx1 + my1 + nz1 = 0



(15.46)

Eliminating (x1, y1, z1), r1 from (15.43), (15.44), (15.45) and (15.46) we get, a h g l



h b f m

g f c n

l m =0 n o

Simplifying this we get,

Al2 + Bm2 + Cn2 + 2Fmn + 2Gnl + 2Hlm = 0

M15_ANALYTICALGEOMETRY_ISBN_CH15.indd 25

(15.47)

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15-26

Analytical Geometry

where A, B, C, F, G, H are the cofactors of a, b, c, f, g, h in the determinant a h g



h b f

g f c

Hence (15.47) is the required condition for the plane (15.40) to touch the cone.

15.7  Reciprocal Cone 15.7.1  Equation of the Reciprocal Cone Let us now find the equation of the cone reciprocal to the cone

ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0.

(15.48)

Let a tangent plane to the cone be lx + my + nz = 0

(15.49)

Al2 + Bm2 + Cn2 + 2Fmn + 2Gnl + 2Hlm = 0

(15.50)

Then we have the condition

where A, B, C, F, G, H are the cofactors of a, b, c, f, g, h in the determinant a h g



h b f

g f c

The equation of the line through the vertex (0, 0, 0) of the cone (15.48) and normal to the tangent plane (15.49) are x y z = = = t l m n



(15.51)

The locus of (4) which is got by eliminating l, m, n from (15.50) and (15.51) is

Ax 2 + By 2 + Cz 2 + 2 Fyz + 2Gzx + 2 Hxy = 0

(15.52)

This is the equation of the reciprocal cone.

Note 15.7.1.1: If we find the reciprocal cone of (15.52) we get the equation of cone as ax 2 + by 2 + cz 2 + 2 fyz + 2 gzx + 2hxy = 0.

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15-27

Cone

Definition 15.7.1.2: Two cones are said to be reciprocal cones of each other if each one is the locus of the normal through the vertex to the tangent planes of the other.

15.7.2  Angle between Two Generating Lines in Which a Plane   Cuts a Cone Let the equation of the cone and the plane be

ax 2 + by 2 + cz 2 + 2 fyz + 2 gzx + 2hxy = 0.

(15.53)

lx + my + nz = 0

(15.54)

and

x y z = = . l m n Since this line lies on the plane we have,

Let one of the lines of the section be



al 2 + bm2 + cn2 + 2 fmn + 2ngl + 2hlm = 0

(15.55)



ul + vm + wn = 0

(15.56)

from (15.56)

l=

−( vm + wn) (u ≠ v ) u

Substituting this in (15.55) we get,

a( vm + wn)2 ( vm + wn) vm + wn + bm2 + cn2 − 2 gn + 2 fmn − 2hm =0 2 u u u



(i.e.)  (av2 + bu2 - 2huv)m2 + 2(avw - guv - fu2 - huw)mn



+ (aw2 + cu2 - 2guw)n2 = 0

This is a quadratic equation in

(15.57)

m (assume n ≠ 0). n

m m m say 1 and 2 . n1 n2 n Hence the given plane intersects the cone in two lines namely, There are two values for



x y z x y z = = and = = l1 m1 n1 l2 m2 n2

where

l1 = −

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vm1 + wn1 u

l2 = −

vm2 + wn2 u

4/22/2013 8:25:54 PM

15-28

Analytical Geometry

Also since

m1 m and 2 are the roots of the equation (15.57) n1 n2 m1 m2 m1n2 + m2 n1 + = n1 n2 n1n2 =

and

m1m2 aw 2 + cu 2 − 2 guv = 2 n1n2 av + bu 2 − 2huv l1l2 =



−2( avw − guv + fu 2 − huw ) av 2 + bu 2 − 2huv

∴

v 2 m1m2 + w 2 n1n2 + vw ( m1n2 + m2 n1 ) u2

 m m  l1l2 1  v 2 m1m2 = 2 + w 2 + vw  1 + 2   n1n2 u  n1n2  n1 n2     v 2 ( aw 2 + cu 2 − 2 guw ) + w 2 − 1  av 2 + bu 2 − 2huv  = 2 u  avw − guv + fu 2 − huw   2vw av 2 + bu 2 − 2huv  



cv 2 + bw 2 − 2 fwv av 2 + bu 2 − 2huv l1l2 m1m2 ∴ 2 = bw + cv 2 − 2 fvw cu 2 + aw 2 − 2 gwu n1n2 = 2 av + bu 2 − 2huv m1n2 + m2 n1 = −2( avw − guv + fu 2 − huw ) =





=

=

( m1n2 + m2 n1 )2 − 4 m1m2 n1n2 ( avw − guv + fu 2 − huv )2 −  ±2   2 2 2 2  4( av + bu − 2huv )( aw + cu − 2 guw )  m1n2 − m2 n1 1

± 2u[ −( Au 2 + Bv 2 + Cw 2 + 2 Fvw + 2Gwu + 2 Huv )]2 m n − m2 n1 = 1 2 ± 2uP

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Cone

15-29

where P2 = -(Au2 + Bv2 + Cw2 + 2Fvw + 2Gwu + 2Huv) and A, B, C, F, G, H a are the cofactors of a, b, c, f, g, h in the determinant h g It follows by symmetry

g f. c

l1l2 bw 2 + cv 2 − 2 fvw



m1m2 cu + aw 2 − 2 gwu n1n2 = 2 av + bu 2 − 2huv



=

=

h b f

2

m1n2 − m2 n1 n1l2 − n2 l1 l1m2 − l2 m1 = = ± 2up ± 2vp ± 2wp

If q is the acute angle between the lines then

cos q = l1l2 + m1m2 + n1n2



(i.e.)



sin q (l1m2 − l2 m1 ) + ( m1n2 − m2 n1 )2 ( n1l2 − n2 l1 )2 2

cos q sin q = ( a + b + c)(u 2 + v 2 + w 2 ) − f (u, v, w ) ± 2 P u 2 + v 2 + w 2 ∴ tan q =

±2 P u 2 + v 2 + w 2 ( a + b + c)(u 2 + v 2 + w 2 ) − f (u, v, w )

Note 15.7.2.1: If the two lines are perpendicular then (a + b + c) (u2 + v2 + w2) - f (u, v, w) = 0.



(i.e.)  f (u, v, w) = (a + b + c) (u2 + v2 + w2)

Note 15.7.2.2: If the lines of intersection are coincident then q  = 0.

∴2 P u 2 + v 2 + w 2 = 0

∴ P = 0 since u2 + v2 + w2 ≠ 0 P 2 = 0

or

Au2 + Bv2 + Cw2 + 2Fvw + 2Gwu + 2Huv = 0

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15-30

Analytical Geometry

a h g u



h b f v

g u f v =0 c w w o

This is the condition for the plane ux + vy + wz = 0 to be a tangent plane to the cone.

15.7.3  Condition for Mutually Perpendicular Generators of the Cone

f(x, y, z) = ax2 + by2 + cz2 + 2fyz + 2gzx + 2hxy = 0

We have seen that the condition for the plane ux + vy + wz = 0 cut the cone in two perpendicular generators is that

(a + b + c)(u2 + v2 + w2) - f(u, v, w) = 0

(15.58)

If there is a third generator which is perpendicular to the above two lines of intersection then it must be a normal to the plane ux + vy + wz = 0. Therefore, its equations are x y z = = u v w Since (15.59) is a generator of the cone f (x, y, z) = 0, we get





f (u, v, u) = 0

(15.59)

(15.60)

(15.58) and (15.60) holds if and only if a + b + c = 0. Therefore, the condition for the cone to have three mutually perpendicular generators is a + b + c = 0.

Example 15.7.1 Find the angle between the lines of section of the plane 3x + y + 5z = 0 and the cone 6yz - 2zx + 5xy = 0.

Solution Let the equations of the line of section be x y z = = = r l m n As this line lies on the plane and also on the cone we get



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3l + m + 5n = 0

(15.61)

(15.62)

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Cone

6mn - 2nl + 5lm = 0



15-31

(15.63)

m = - (3l + 5n)

From (15.62)

Substituting this in (15.63) we get,

-6n(3l + 5n) - 2nl + 5l(3l + 5n) = 0

  (i.e.)  15l 2 + 45ln + 30n2 = 0

(i.e.)  l 2 + 3ln + 2n2 = 0



(l + n) (l + 2n) = 0



l + n = 0  or  l + 2n = 0

Solving l + n = 0 and 3l + m + 5n = 0 we get, l m n = = 1 2 −1



Solving l + 2n = 0 and 3l + m + 5n = 0 we get, l m n = = −2 1 1



Therefore, the direction ratios of the two lines are 1, 2, -1 and 2,-1, -1. If q  is the angle between the lines cosq =

2 − 2 +1

1 = . 6 6 6

 1 Therefore, the acute angle between the lines is q = cos −1   .  6

Example 15.7.2 Prove that the angle between the lines given by x + y + z = 0, ayz + p bzx + cxy = 0 is if a + b + c = 0. 2

Solution The plane

x+y+z=0

(15.64)

ayz + bzx + cxy = 0

(15.65)

will cut the cone

in two lines through the vertex (0, 0, 0).

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15-32

Analytical Geometry

The equations of the lines of the section are x y z = = l m n



(15.66)

where l, m, n are the direction ratios of the lines. Since the line given by (15.66) lies on the plane and the cone

l + m + n = 0

(15.67)



amn + bnl + clm = 0

(15.68)

Substituting n = -(l + m) in (15.67) we get

-am(l + m) - b(l + m)l + clm = 0



(i.e.)  bl 2 + (a + b - c)lm + am2 = 0



 l2  1 or  b  2  + ( a + b − c) + a = 0 m m 

If l1, m1, n1 and l2, m2, n2 are the direction ratios of the two lines we get, l1l2 a = m1m2 b



 ∴

l1l2 m1m2 = a b

Similarly we can show that m1m2 n1n2 = b c



∴

l1l2 m1m2 n1n2 = = =t a b c



l1l2 = ta



m1m2 = tb



n1n2 = tc

say

If q is the angle between the lines

cosq = l1l2 + m1m2 + n1n2 = t(a + b + c) = 0 (∵ a + b + c = 0)



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∴ q =

p 2

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Cone

Example 15.7.3 Prove that the cones ax2 + by2 + cz2 = 0 and

15-33

x2 y2 z2 + + = 0 are reciprocal. a b c

Solution The equation of the reciprocal cone ax2 + by2 + cz2 = 0 is

Ax2 + By2 + Cz2 + 2Fyz + 2Gzx + 2Hxy = 0

a where A, B, C are the cofactors of a, b, c in h g

h b f

g f. c



(i.e.)  A = bc - f 2  F = gh - af = 0



B = ca - g2  G = hf - bg = 0



C = ab - h2  H = fg - ch = 0

The equation of the reciprocal cone is bcx2 + cay2 + abz2 = 0 or

x2 y2 z2 + + = 0. a b c

x2 y2 z2 Similarly, we can show that the reciprocal cone of + + = 0 is a b c ax2 + by2 + cz2 = 0. Therefore, the two given cones are reciprocal to each other.

Example 15.7.4 Show that the equation the coordinate planes.

fx ± gy ± hz = 0 represents a cone which touches

Solution

fx ± gy ± hz = 0

(   

fx ± gy

) = ( − hz ) 2

2

fx + gy − hz = ±2 fgxy Squaring (fx + gy - hz)2 = 4fgxy

(i.e.)  f 2x2 + g2y2 + h2z2 + 2fgxy - 2hgyz - 2hfzx = 0

This being a homogeneous equation of second degree in x, y, z, it represents a cone. When this cone meets the plane x = 0 we get, (gy - hz)2 = 0.

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15-34

Analytical Geometry

Hence the above cone is cut by the plane x = 0 in coincident lines and hence x = 0 touches the cone. Similarly, y = 0, z = 0 also touch the cone.

Exercises 3 1.



Find the angle between the lines of the section of the planes and cones: (i) x + 3y - 2z = 0, x2 + 9y2 - 4z2 = 0 (ii) 6x - 10y - 7z = 0, 108x2 - 20y2 - 7z2 = 0 3 16 , (ii) cos −1 Ans.: (i) cos −1 21 65

2.

Show that the angle between the lines in which the plane x + y + z = 0 cuts the p 1 1 1 if + + = 0. cone ayz + bzx + cxy = 0 is 3 a b c

3.

Prove that the equation a2x2 + b2y2 + c2z2 - 2bcyz - 2cazx - 2abxy = 0 represents a cone which touches the coordinate plane.

4.

If

 5. 

x y z = = represents one of the generators of the three mutually 1 2 3 perpendicular generators of the cone 5 yz − 8 zx − 3 xy = 0 then find the other two. x y z x y z Ans.: = = ; = = 1 1 −1 5 −4 1 y If x = = z represents one of the three mutually perpendicular generators of 2 the cone 11 yz + 6 zx − 14 xy = 0 then find the other two. x y z x y z = ; = = Ans.: = 2 −3 4 −11 2 7

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Chapter

16

Cylinder

16.1  Definition The surface generated by a variable line which remains parallel to a fixed line and intersects a given curve (or touches a given surface) is called a cylinder. The variable line is called the generator, the fixed straight line is called the axis of the cylinder and the given curve is called the guiding curve of the cylinder.

16.2  Equation of a cylinder with a given generator and a given guiding curve Let us find the equation of the cylinder whose generators are parallel to the line x y z = = l m n



(16.1)

and whose guiding curve is the conic ax 2 + 2hxy + by 2 + 2 gx + 2 fy + c = 0; z = 0



(16.2)

Let (a, b, g ) be any point on the cylinder. Then the equations of a ­generator are x −a y − b z −g = = l m n



(16.3)

Let us find the point where this line meets the plane z = 0. When z = 0, x −a y − b −g = = . l m n

x =a −

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lg mg ,y=b− ,z=0 n n

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16-2

Analytical Geometry

lg mg   , 0 . This point is  a − , b −  n n  When the generator meets the conic, this point lies on the conic. 2

lg  lg   mg  mg     ∴ a  a −  + 2h  a −   b − + b b −      n n n  n 

2

lg  mg    + 2g a −  + 2 f  b −  +c = 0    n n  i.e. a (na - lg )2 + 2h (na - lg ) (nb - mg ) + b (nb - mg )2 + 2gn (na - lg ) + 2fn(nb - mg ) + cn2 = 0 The locus of the point (a, b, g ) is a(nx - lz)2 + 2h(nx - lz)(ny - mz) + b(ny - mz)2 + 2gn(nx - lz)    + 2fn(ny - mz) + cn2 = 0 This is the required equation of the cylinder.

Note 1: If the generators are parallel to z-axis l = 0, m = 0, n = l then the ­equation of the cylinder becomes, f(x, y) = ax2 + 2hxy + by2 + 2gx + 2fy + c = 0

Note 2: The equation f(x, y) = 0 in space represents a cylinder whose generators are parallel to z-axis.

16.3  Enveloping Cylinder The locus of the tangent lines drawn to a sphere and parallel   to a given line Let the given sphere be

x2 + y2 + z2 = a2

(16.4)

Let the given line be x y z = = l m n Let (a, b, g ) be any point on the locus. Then any line through (a, b, g ) parallel to line (16.5) is



x −a y − b z −g = = = r l m n Any point on this line is (a + lr, b + mr, g + nr)



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(16.5)

(16.6)

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Cylinder

16-3

If the point lies on the sphere (16.4), then (a + lr)2 + ( b + mr)2 + (g + nr)2 = a2 (i.e.) r2(l2 + m2 + n2) + 2r(al + bm + g n) + a 2 + b 2 + g 2 - a2 = 0 (16.7)



This is a quadratic equation in r giving the two values for r corresponding to two points common to the sphere and the line. If the line is a tangent then the two values of r must be equal and hence the discriminant must be zero.

\ (a l + b m + g n)2 = (l 2 + m2 + n2) (a 2 + b 2 + g 2 - a2)

The locus (a, b, g ) is (lx + my + nz)2 = (l2 + m2 + n2) (x2 + y2 + z2 - a2)



which is a cylinder. This cylinder is called the enveloping cylinder of the sphere.

Enveloping cylinder as a limiting form of an enveloping cone x y z = = = r be the axis of the enveloping cylinder. Any point on this l m n line is (lr, mr, nr). Let this point be the vertex of the enveloping cone. Then the equation of the enveloping cone is T 2 = SS1. (i.e.) (lrx + mry + nrz - a2)2 = (x2 + y2 + z2 - a2) (l2r2 + mr2 + n2r2 - a2) Let

  a2  a2  (i.e.)  lx + my + nz −  = ( x 2 + y 2 + z 2 − a 2 )  l + m + n − 2  r r    When r → ∞, the above equation becomes,

(lx + my + nz)2 = (x2 + y2 + z2 - a2) (l2 + m2 + n2)

which is the equation to the enveloping cylinder.

16.4  Right Circular Cylinder A right circular cylinder is a surface generated by a straight line which remains parallel to a fixed straight line at a constant distance from it. The fixed straight line is called the axis of the cylinder and the constant distance is called the radius of the cylinder.

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16-4

Analytical Geometry

The equation of a right circular cylinder whose axis is the straight line x −a y − b z −g = = and whose radius is a. l m n A Q (a, b,g ) a L

a

P(x, y, z)

A′

Let P (x, y, z) be any point on the cylinder. Let AA′ be the axis of the cylinder. Draw PL perpendicular to the axis and PL = a. Let Q(a, b, g ) be a point on the axis of the cylinder.

PQ2 = (x - a)2 + (y - b )2 + (z - g )2 QL = Projection of PQ on this axis of the cylinder (x −a ) l + ( y − b ) m + (z −g ) n = l 2 + m 2 + n2 Also, QL2 = PQ2 - PL2 PQ2 - QL2 = a2

2

(x −a ) l + ( y − b ) m + (z −g ) n 2 (x - a )2 + (y - b )2 + (z - g )2 -   =a 2 2 2 l +m +n   This is the required equation of the right cylinder.

Illustrative Examples Example 16.1 Find the equation of the cylinder whose generators are parallel to the line x y z = = and whose guiding curve is x2 + y2 = 9, z = 1. −1 2 3

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Cylinder

16-5

Solution Let P(x, y, z) be a point on the cylinder.

P(x, y, z)

x y z The equations of the generator through P and parallel to the line = = − 1 2 3 are

x − x1 y − y1 z − z1 = = −1 2 3

(16.8)

x2 + y2 = 9, z = 1

(16.9)

The guiding curve is

When the generator through P meets the guiding curve, x − x1 y − y1 z − z1 = = −1 2 3



\ x = x1 −

1 − z1 3 x1 + z1 − 1 2 (1 − z1 ) 3 y1 − 2 z1 + 2 , y = y1 + = = 3 3 3 3

Since this point lies on the curve (16.9),

(3 x1 + z1 − 1)2 + (3 y1 − 2 z1 + 2)2 = 81

The locus of ( x1 , y1 , z1 ) is (3x + z - 1)2 + (3y - 2z + 2)2 = 81

(i.e.) 9x2 + 9y2 + 5z2 + 6xz - 12yz - 6x + 12y - 10z - 76 = 0

This is the equation of the required cylinder.

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16-6

Analytical Geometry

Example 16.2 Find the equation of the cylinder which intersects the curve ax2 + by2 + cz2 = 1, lx + my + nz = p and whose generators are parallel to z-axis.

Solution The equation of the guiding curve is ax2 + by2 + cz2 = 1, lx + my + nz = p



(16.10)

Since the generators are parallel to z-axis the equation of the cylinder is of the form f (x, y) = 0. The equation of the cylinder is obtained by eliminating z in equation (16.10) z=



lx + my − p −n

Substituting this in ax2 + by2 + cz2 = 1, we get,  lx + my − ax 2 + by 2 + c   −n



2

p  = 1

\ (an2 + cl2) x2 + (bn2 + cm2) y2 + 2clmxy - 2cpmy + cp2 - n2 = 0

This is the equation of the required cylinder.

Example 16.3 Find the equation of the cylinder whose generators are parallel to the line x y z = = and whose guiding curve is the ellipse x2 + 2y2 = 1, z = 3. 1 −2 3

Solution The equation to the guiding curve is x2 + 2y 2 = 1, z = 3



(16.11)

Let ( x1 , y1 , z1 ) be a point on the cylinder. Then the equations of the generator x − x1 y − y1 z − z1 = = through P( x1 , y1 , z1 ) are −2 1 3 When this line meets the plane z = 3, we have,

x − x1 y − y1 3 − z1 = = −2 1 3

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16-7

Cylinder



\x = x1 +

3 − z1 3 x1 − z1 + 3 2 (3 − z1 ) 3 y1 + 2 z1 − 6 , y = y1 − = = 3 3 3 3

This point lies on the curve x2 + 2y2 = 1. 2

2

 3 y1 + 2 z1 − 6   3x − z + 3 \ 1 1  = 1  + 2   3 3 (3 x1 − z1 + 3)2 + 2 (3 y1 + 2 z1 − 6)2 = 9

9 x12 + 18 y12 + 9 z12 + 24 y1 z1 − 6 z1 x1 + 18 x1 − 72 y1 − 54 z1 + 72 = 0

The locus of ( x1 , y1 , z1 ) is

9x2 + 18y2 + 9z2 + 24yz - 6zx + 18x - 72y - 54z + 72 = 0 (i.e.) 3x2 + 6y2 + 3z2 + 8yz - 2zx + 6x - 24y - 18z + 24 = 0

Example 16.4 Find the equation of the surface generated by the straight line y = mx, z = nx x2 y2 + = 1, z = 0 . a2 b2

and intersecting the ellipse

Solution The given line y = mx, z = nx can be expressed in symmetrical form as x y z = = 1 m n



(16.12)

Let P( x1 , y1 , z1 ) be any point on the cylinder. Then the equations of the generator through P are x − x1 y − y1 z − z1 = = m 1 n



This meets the curve When z = 0,

x2 y2 + = 1, z = 0 a2 b2

x − x1 y − y1 − z1 = = m n 1

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16-8

Analytical Geometry



\ x = x1 −

z1 nx1 − z1 mz ny − mz1 = , y = y1 − 1 = 1 n n n n

This point lies on the curve

x2 y2 + = 1. a2 b2 2

2



1  nx1 − z1  1  ny − mz1  + 2 1  = 1  2    n n a b 



b 2 ( nx1 − z1 )2 + a 2 ( ny1 − mz1 )2 = n2 a 2 b 2

The locus of ( x1 , y1 , z1 ) is b 2 ( nx − z )2 + a 2 ( ny − mz )2 = n2 a 2 b 2



which is the equation of the required cylinder.

Example 16.5 Find the equation of the circular cylinder whose generating lines have the direction cosines l, m, n and which passes through the circumference of the fixed circle x2 + y2 = a2 on the xoz plane.

Solution Let P( x1 , y1 , z1 ) be any point on the cylinder. Then the equations of the generator x − x1 y − y1 z − z1 = = . l m n This meets the plane y = 0.

through P are

x − x1 − y1 z − z1 = = l m n



\ x = x1 −

ly1 mx1 − ly1 ny mz − ny1 = , z = z1 − 1 = 1 m m m m

This point lies on the curve

x2 + y2 = a2 2



2

 mx1 − ly1   mz1 − ny1  2 +  = a  m   m ( mx1 − ly1 )2 + ( mz1 − ny1 )2 = m2 a 2

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Cylinder

16-9

The locus of ( x1 , y1 , z1 ) is ( mx − ly )2 + ( mz − ny )2 = m2 a 2



which is the required equation of the cylinder.

Example 16.6 Find the equations of the right circular cylinder of radius 3 with equations of axis as

x −1 y − 3 z − 5 = = . 2 2 −1

Solution The equations of the axis are x −1 y − 3 z − 5 = = 2 2 −1



(1, 3, 5) is a point on the axis. 2, 2, -1 are the direction ratios of the axis. \ direction cosines are

2 2 −1 , , 3 3 3 A

(1, 3, 5) Q L

3

P(x1, y1, z1)

A′

Let P ( x1 , y1 , z1 ) be any point on the cylinder. QL = Projection of PQ on the axis = ( x − x1 )l + ( y − y1 )m + ( z − z1 )n 2 2 1 = ( x1 − 1) + ( y1 − 3) − ( z1 − 5) 3 3 3 2 x1 + 2 y1 − z1 − 3 = 3

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16-10

Analytical Geometry

Also, PQ2 = QL2 + LP2 2

 2 x + 2 y1 − z1 − 3  (i.e.)  ( x1 − 1)2 + ( y1 − 3)2 + ( z1 − 5)2 =  1  + 9  3 The locus of ( x1 , y1 , z1 ) is 9(x2 - 2x + 1 + y2 - 6y + 9 + z2 - 10z + 25)   = 4x2 + 4y2 + z2 + 9 + 8xy - 4xz - 12x - 4yz - 12y + 6z + 81 (i.e.)   5x2 + 5y2 + 8z2 - 8xy + 4xz + 4yz - 6x - 42y - 96z + 225 = 0 This is the equation of the required cylinder.

Example 16.7 Find the equation of the right circular cylinder whose axis is x = 2y = - z and radius 4.

Solution The equations of the axis of the cylinder are x y z = = 1 12 −1

(i.e.)

x y z = = 2 1 −2 A

Q L

P(x1, y1, z1)

A′

Let P ( x1 , y1 , z1 ) be any point on the cylinder. The equations of the generator through P are

x − x1 y − y1 z − z1 = = −2 2 1

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Cylinder

The direction cosines of the axis are

16-11

2 1 −2 , , . 3 3 3

QL = Projection of PQ on this axis. 2 1 −2 = ( x1 − 0) + ( y1 − 0) + ( z1 − 0) 3 3 3 2 x1 + y1 − 2 z1 3 Also, PQ2 = QL2 + LP 2 =

2

 2 x + 2 y1 − z1 − 3  x12 + y12 + z12 =  1  + 16  3



The locus of ( x1 , y1 , z1 ) is 9( x 2 + y 2 + z 2 ) = (2 x1 + 2 y1 − z1 − 3)2 + 144 (i.e.)

5x2 + 5y2 + 8z2 - 4xy + 4yz + 8zx - 144 = 0

Example 16.8 Find the equation of the cylinder whose generators have direction cosines l, m, n and which passes through the circle x2 + z2 = a2, y = 0.

Solution A

Q L

P(x1, y1, z1)

A′

Let P ( x1 , y1 , z1 ) be any point on the cylinder. The equation of the generators through P are

x − x1 y − y1 z − z1 = = l m n

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16-12

Analytical Geometry

This line meets the curve y = 0, x2 + z2 = a2 When y = 0,

x − x1 − y1 z − z1 = = l m n

\ x = x1 −

ly1 mx1 − ly1 = , m m

z = z1 −

ny1 mz1 − ny1 = m m 2

2

 mx − ly1   mz1 − ny1  2 + This point lies on x2 + z2 = a2 or  1  = a .  m   m The locus of ( x1 , y1 , z1 ) is ( mx − ly )2 + ( mz − ny )2 = m2 a 2



This is the equation of the required cylinder.

Example 16.9 Find the equation of the right circular cylinder whose axis is and passes through the point (0, 0, 3).

x − 2 y −1 z = = 2 1 3

Solution The equations of the axis of the cylinder are x − 2 y −1 z = = 2 1 3



2 1 3   + (1 − 0) + (0 − 3) r2 = 4 + 1 + 9 - (2 − 0)  14 14 14   4 +1− 9 = 14 −    14  =

2

2

196 − 16 180 90 = = 14 14 7

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16-13

Cylinder

(2, 1, 0) P(0, 0, 3)

r

Let P ( x1 , y1 , z1 ) be any point on the cylinder, then 90 = ( x1 − 2)2 + ( y1 − 1)2 + ( z1 − 0)2 7 2 2 1 3   − ( x1 − 2) + ( y1 − 1) + ( z1 − 3)  14 14 14   (2 x1 + y1 − 3 z1 − 14)2 90 = ( x1 − 2)2 + ( y1 − 1)2 + z12 − 14 7 90 × 14 = 14 [( x1 − 2)2 + ( y1 − 1)2 + z12 ] − (2 x1 + y1 − 3 z1 − 14)2 7 The locus of ( x1 , y1 , z1 ) is

10x2 + 13y2 + 5z2 - 6yz - 12zx - 4xy - 36x - 18y + 30z - 135 = 0 This is the equation of the required cylinder.

Example 16.10 Find the equation to the right circular cylinder which passes through circle x2 + y2 + z2 = 9, x - y + z = 3.

Solution For the right circular cylinder, the guiding curve is the circle x2 + y2 + z2 = 9, x - y + z = 3. Therefore, the direction ratios of the axis of the cylinder are 1, -1, 1. Let P ( x1 , y1 , z1 ) be any point on the cylinder. Then the equations of the generator through P are

x − x1 y − y1 z − z1 = = = r l m n

M16_ANALYTICALGEOMETRY_ISBN_CH16.indd 13

(16.13)

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16-14

Analytical Geometry

Any point on this line is ( r − x1 , − r + y1 , r + z1 ) . If this point lies on the circle, then ( r − x1 )2 + ( − r + y1 )2 + ( r + 3)2 = 9

(16.14)

r + x1 − ( − r + y1 ) + r + z1 = 3

(16.15)

x + y + z + 2r ( x1 − y1 + z1 ) + 3r = 9

(16.16)

( x1 − y1 + z1 ) + 3r = 3

(16.17)

and (i.e.)

2 1

2 1

2 1

2



Eliminating r from (16.16) and (16.17) we get 2 1 ( x12 + y12 + z12 ) + (3 − x1 + y1 − z1 )( x1 − y1 + z1 ) + 3. (3 − x1 + y1 − z1 )2 = 9 3 9 Simplifying, the locus of ( x1 , y1 , z1 ) is x2 + y2 + z2 + xy + yz + zx - 9 = 0



which is the equation of the required cylinder.

Example 16.11 Find the equation to the right circular cylinder of radius a whose axis passes through the origin and makes equal angles with the coordinate axes.

Solution Let l, m, n be the direction cosines of the axis of the cylinder. A

O L

a

P(x1, y1, z1)

A′

Given: l = m = n \l=m=n=

l 2 + m 2 + n2 3

M16_ANALYTICALGEOMETRY_ISBN_CH16.indd 14

=

1 3

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Cylinder

16-15

The axis passes through the origin. Let P ( x1 , y1 , z1 ) be any point on the cylinder. OL = Projection of OP on the axes 1 1 −2 = ( x1 − 0) + ( y1 − 0) + ( z1 − 0) 3 3 3 =

x1 + y1 + z1 3 OP2 = OL2 + LP2

(

x12 + y12 + z12

)

2

2

 x + y1 + z1  2 = 1  + a  3

\( x12 + y12 + z12 )2 − 3( x12 + y12 + z12 − 3a 2 ) = 0 The locus of ( x1 , y1 , z1 ) is 2x2 + 2y2 + 2z2 - 2xy + 2yz - 2zx - 3a2 = 0



This is the equation of the required cylinder.

Example 16.12 Find the equation to the right circular cylinder described on the circle through the points A(1, 0, 0), B(0, 1, 0) and C(0, 0, 1) as the guiding curve x2 + y2 + z2 - yz - zx - xy = 1.

Solution N

P

C 1,1,1 2 2 2

The equation of the sphere OABC is

x2 + y2 + z2 - x - y - z = 0

(16.18)

The equation of the plane ABC is

M16_ANALYTICALGEOMETRY_ISBN_CH16.indd 15

x + y + z = 1

(16.19)

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16-16

Analytical Geometry

Therefore, the equation of the circle ABC is  x 2 + y 2 + z 2 − x − y − z = 0, x + y + z = 1



(16.20)

 1 1 1 The centre of the sphere is  , ,  .  2 2 2

1 1 1 3 + + = 4 4 4 2 CN = Perpendicular from the centre on the plane x + y + z = 1 r=

=

1 2

+ 12 + 12 − 1 3

=

1 2 3 NP2 = CP2 - CN2 = ∴NP =

3 1 2 − = 4 12 3 2 3

The equations of the line CN are x − 12 y − 12 z − 12 = = 1 1 1



which is the axis of the cylinder. The direction ratios of the axis are 1, 1, 1. The direction cosines of the axis are

1 3

,

1 3

,

1 3

.

Q L

M16_ANALYTICALGEOMETRY_ISBN_CH16.indd 16

P

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16-17

Cylinder

 1 1 1 P is  , ,  .  2 2 2 Q is ( x1 , y1 , z1 ). 2 3

 PL =

2

2

1  1  1  PQ =  x −  +  y −  +  z −   2  2  2 = x12 + y12 + z12 − x1 − y1 − z1 +

2

3 4

QL = Projection of PQ on the axis 1  = x−   2 =

2

1  +y−   2 3

1

x1 , y1 , z1 −

2

1  +z−   2 3

1

2

1 3

1 3

3

PQ = QL + LP2 2

2

2

x12 + y12 + z12 − x1 − y1 − z1 +

3  x1 + y1 + z1 − 23  2 =  + 3 4  3

The locus of ( x1 , y1 , z1 ) is 2



3  3  3  x2 + y2 + z2 − x − y − z +  =  x + y + z −  + 2  4  2



(i.e.) x2 + y2 + z2 - yz - zx - xy = 1

which is the required equation of the cylinder.

Example 16.13 Find the equation of the enveloping cylinder of the sphere x2 + y2 + z2 - 2x + 4y = 1 having its generators parallel to the line x = y = z.

Solution Let P ( x1 , y1 , z1 ) be any point on a tangent which is parallel to the line

M16_ANALYTICALGEOMETRY_ISBN_CH16.indd 17

x = y = z

(16.21)

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16-18

Analytical Geometry

Therefore, the equation of the tangent lines are x − x1 y − y1 z − z1 = = 1 1 1



(16.22)

Any point on this line is ( x1 + r , y1 + r , z1 + r ). This point lies in this sphere x2 + y2 + z2 - 2x + 4y - 1 = 0



(16.23)



\ ( x1 + r )2 + ( y1 + r )2 + ( z1 + r )2 − 2 ( x1 + r ) + 4 ( y1 + r ) − 1 = 0



(i.e.) 3r 2 + 2r ( x1 + y1 + z1 − 1) + ( x12 + y12 + z12 − 2 x1 + 4 y1 − 1) = 0

If equation (16.22) touches the sphere of equation (16.23), then the two values of r of this equation are equal. \Discriminant = 0 \4( x1 + y1 + z1 − 1)2 − 12( x12 + y12 + z12 − 2 x1 + 4 y1 − 1) = 0 (i.e.) 3( x12 + y12 + z12 − 2 x1 + 4 y1 − 1) − ( x1 + y1 + z1 − 1)2 = 0 On simplifying we get the locus of ( x1 , y1 , z1 ) as

x2 + y2 + z2 - yz - zx - 2x + 7y + z - 2 = 0

which is the required equation.

Exercises 1.

Find the equation of the cylinder, whose guiding curve is x2 + z2 - 4x - 2z + 4 = 0, y = 0 and whose axis contains the point (0, 3, 0). Find also the area of the section of the cylinder by a plane parallel to the xz plane.

2.

Find the equation of the cylinder, whose generators are parallel to the line x y z = = and passing through the curve x2 + 2y2 = 1, z = 0. 1 −2 3 Prove that the equation of the cylinder with generators parallel to z-axis and passing through the curve ax2 + by2 = 2cz, lx + my + nz = p is n (ax2 + by2) + 2c (lx + my) - 2pc = 0.

3.

4.

Find the equation of the cylinder, whose generators are parallel to the line x y z = = and passes through the curve x2 + y2 = 16, z = 0. 1 2 3

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Cylinder

16-19

5.

Find the equation to the cylinder with generators parallel to z-axis which passes through the curve of intersection of the surface represented by x2 + y2 + 2z2 = 12 and lx + y + z = 1.

6.

Find the equation of the cylinder, whose generators intersect the conic ax2 + 2hxy + by2 = 1, z = 0 and are parallel to the line with direction cosines l, m, n.

7.

y2 1 A cylinder cuts the plane z = 0 with curve x 2 + = and has its axis parallel 4 4 to 3x = -6y = 2z. Find its equation.

8.

A straight line is always parallel to the yz plane and intersects the curves x2 + y2 = a2, z = 0 and x2 = az, y = 0. Prove that it generates the surface x4y2 = (x2 - az)2 (a2 - x2).

9.

Find the equation of a right circular cylinder of radius 2 whose axis passes through (1, 2, 3) and has direction cosines proportional to 2, 1, 2.

10.

Find the equation of the right circular cylinder of radius 2 whose axis passes through (1, 2, 3) and has direction cosines proportional 2, -3, 6.

11.

Find the equation of the right circular cylinder of radius 1 with axis as x −1 y z − 3 = = . 2 3 1

12.

Find the equation of the right circular cylinder whose generators are parallel x y z to = = and which passes through the curve 3x2 + 4y2 = 12, z = 0. 2 5 3

13.

Find the equation of the right circular cylinder of radius 4 whose axis is x = 2y = -z.

14.

Find the equation of the right circular cylinder whose guiding curve is the circle through the point (a, 0, 0), (0, b, 0), (0, 0, c).

15.

Find the equation of the enveloping cylinder of the sphere x2 + y2 + z2 - 2x + 4y = 1 having its generators parallel to the line x = y = z.

16.

Find the enveloping cylinder of the sphere x2 + y2 + z2 - 2y - 4z = 11 having its generators parallel to the line x = -2y = 2z.

17.

Find the equation of the right cylinder which envelopes a sphere of centre (a, b, c) and radius r and its generators parallel to the direction l, m, n.

Answers 1.

10x2 + 5y2 + 13z2 + 12xy + 4xz + 6yz - 36x - 30y - 18z + 36 = 0

2.

3x2 + 6y2 + 3z2 - 2xz + 8yz - 3 = 0

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16-20

Analytical Geometry

4.

9x2 + 9y2 + 5z2 - 6xz - 12yz - 144 = 0

5.

3x2 + 3y2 + 4xy - 4x - 4y - 10 = 0

7.

36x2 + 9y2 + 17z2 + 6yz - 48xz - 9 = 0

9.

5x2 + 8y2 + 5z2 - 4yz - 8zx - 4xy + 22x - 16y - 14z - 10 = 0

10.

45x2 + 40y2 + 13z2 + 36yz - 24zx + 12xy - 42x - 280y - 126z + 294 = 0

11.

10x2 + 5y2 + 13z2 - 12xy - 6yz - 4zx - 8x + 30y - 74z + 59 = 0

12.

27x2 + 36y2 + 112z2 - 36xz - 120yz - 180 = 0

13.

5x2 + 8y2 + 5z2 - 4xy + 4yz + 8zx - 144 = 0

14.

1 1 2  1 2 2  2 + 2 + 2  ( x + y + z − ax − by − cz ) a b c 1 1 1 1 1 1  =  + + − 1  + + − 2 a b c a b c 

15.

x2 + y2 + z2 - xy - yz - zx - 4x + 5y - z - 2 = 0

16.

5x2 + 8y2 + 8z2 + 4xy + 2yz - 4xz + 4x - 18y - 36z = 99

17.

(l 2 + m2 + n2 )[( x − a)2 + ( y − b)2 + ( z − c)2 − r 2 ] = [l ( x − a) + m( y − b) + n( z − c)]2

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Index A asymptotes, 8-28–8-33 angle between, 8-30–8-33 equations of hyperbola, 8-28–8-29 auxillary circle, 7-5 axis of cylinder, 16-1 axis of parabola, 6-3 C cartesian coordinates, 9-3 centre of ellipse, 7-3 centroid of tetrahedron, 11-8 centroid of triangle, 1-4, 11-7 chord in terms of midpoint, 6-14–6-15 chord joining two points, 6-16–6-17 chord of circle, 4-9–4-10 chord of contact of tangents, 4-5–4-6 circle, 4-1–4-36, 9-9–9-16 coaxal system, 5-3–5-4 definition of, 4-1 equation of a tangent at, 4-3 equation of, 14-5 examples related to, 4-11–4-36 length of a tangent to a point on, 4-2–4-3 limiting points, 5-4–5-6 orthogonal circle, 5-2–5-3 parametric form of, 4-11 polar equation of, 9-9–9-11 radical axis of, 5-1–5-2 system of, 5-1–5-36 coaxal system, 5-3–5-4, 14-9–14-11 definition of, 5-3–5-4 limiting points of, 5-4

Z01_ANALYTICALGEOMETRY_3604_INDEX.indd 1

combined equation of pair of tangents, 7-52–7-54 common chord of two circle, 5-2 concurrent straight lines, 2-10–2-11 concyclic points, 7-49–7-50, 8-45–8-46 condition for tangency, 6-8–6-9, 7-11–7-12 cone, 15-1–15-34 condition for a general equation of second degree to represent, 15-7–15-11 equation of a cone with a given vertex, 15-1–15-4 equation of a cone with a vertex at origin, 15-4–15-7 conic, 6-1–6-2, 7-1 general equation of second degree and tracing of, 10-1 general equation of, 6-1–6-2 polar equation of, 9-17–9-50 conjugate axis, 8-3 conjugate diameters, 7-54–7-74, 8-35–8-41 definition of, 8-35 equi-conjugate diameters, 7-84–7-85 examples related to, 7-85–7-100, 8-38–8-41 property of, 8-35–8-38 semi-conjugate diameter, 7-81–7-82 conjugate hyperbola, 8-4–8-24 examples related to, 8-5–8-24 conjugate lines, 4-8–4-9, 6-13–6-14, 7-18 condition for, 7-18

4/23/2013 5:10:20 PM

I-2

Index

conormal points, 7-48–7-49, 8-45 coordinate geometry, 1-1–1-27 examples related to, 1-7–1-27 coplanar lines, 13-7–13-9 cylinder, 16-1–16-18 definition of, 16-1 equation with a given generator, 16-1–16-2 D direction cosines, 11-8–11-13 definition of, 11-8 direction ratios, 11-9–11-10 director circle of ellipse, 7-12–7-13 directrix, 6-3 equation of, 6-3 distance form, 2-6–2-7 E eccentric angle, 7-45 equation of chord joining the points at, 7-45–7-46 ellipse, 7-1–7-100 standard equation of, 7-1–7-4 enveloping cone, 15-15–15-16 enveloping cylinder, 16-2–16-3 equation of chord of contact, 6-12 equation of chord, 7-51–7-52 in terms of its middle point, 7-51–7-52 equation of normal, 6-11–6-12, 7-14–7-16 equation of polar, 7-17 equation of tangent, 6-10–6-11, 7-13–7-14 equation to chord of contact, 7-16 equi-conjugate diameters, 7-84–7-85 external point of division, 1-4 F focal distance, 7-4 focus directrix property, 6-5–6-7, 7-1 examples related to, 6-5–6-7, 7-6–7-10

Z01_ANALYTICALGEOMETRY_3604_INDEX.indd 2

G guiding curve of cylinder, 16-1 H homogenous equation of second degree, 3-2 hyperbola, 8-1–8-60 definition of, 8-1 equation in parametric form, 8-3 focus directrix property of, 8-2 important property of, 8-3 latus rectum of, 8-3 parameter of, 8-5 rectangular hyperbola, 8-3 standard equation of, 8-1–8-3 I intercept form, 2-4–2-5 intersection of parabola and circle, 6-19 invariants, 10-3–10-4 L latus rectum, 603 definition of, 6-3 equation of, 6-3 length of, 6-3 limiting points, 5-4–5-6 definition of, 5-4 examples related to, 5-16–5-26 theorems related to, 5-4–5-6 locus of midpoint, 7-80 locus, 1-6–1-7 M major axis of ellipse, 7-3 minor axis of ellipse, 7-3 N non-intersecting lines, 13-13–13-14 normal form, 2-5–2-6

4/23/2013 5:10:21 PM

I-3

Index

number of normals from a point, 6-18–6-19 number of tangents, 6-9 O orthogonal circle, 5-2–5-3 definition of, 5-2 P pair of tangents, 4-10–4-11, 6-13–6-14 parabola, 6-1–6-61 axes and vertex of, 10-10–10-16 forms of, 6-3–6-4 general equation of, 6-2–6-3 important points related to, 6-3 parallel chords, 7-80–7-81 definition of, 7-81 parameter, 6-16 examples related to, 6-41–6-61 parametric form, 2-6–2-7 parametric representation, 6-16 perpendicular tangents, 6-9–6-10 plane, 12-1–12-33 angle between two planes, 12-5 definition of, 12-1 equation of a plane in intercept form, 12-2–12-3 equation of a plane in normal form, 12-3–12-4 equation of planes which bisect the angle between two given planes, 12-10–12-12 examples related to, 12-12–12-33 general equation of a plane passing through a given point, 12-2 general equation of, 12-1–12-2 perpendicular distance from a point on a plane, 12-5–12-7 plane passing through intersection of two given planes, 12-9–12-10

Z01_ANALYTICALGEOMETRY_3604_INDEX.indd 3

plane passing through three given points, 12-7–12-8 point of intersection of normals, 6-18 point of intersection of tangents, 6-17–6-18 polar, 4-7–4-8 polar coordinates, 9-1–9-50 definition of, 9-1–9-3 polar equation, 9-4–9-9 polar of a point, 6-13 definition of, 6-13 pole, 4-7–4-8 definition of, 4-7 position of a point, 7-4–7-5 projection of a line, 11-10 Q quadrilateral, 1-6–1-7 R radical axis, 5-1–5-2 definition of, 5-1 examples related to, 5-6–5-16 radical plane, 14-7–14-9 reciprocal cone, 15-26–15-34 rectangular coordinate axis, 11-1–11-3 rectangular hyperbola, 8-3, 8-41–8-60 definition of, 8-3 properties of, 8-44 results concerning, 8-44–8-45 right circular cone, 15-13–15-21 right circular cylinder, 16-3–16-4 S second directrix, 7-3 second focus, 7-3 section formulas, 1-3–1-7, 11-5–11-7 semi-conjugate diameter, 7-81–7-82 skew lines, 13-9–13-13 slope-intercept form, 2-3–2-4

4/23/2013 5:10:21 PM

I-4

Index

slope-point form, 2-5 sphere, 14-1–14-39 definition of, 14-1 equation of, 14-1–14-2 intersection of two, 14-5 passing through a circle, 14-6 standard equation, 7-1 straight line, 2-1–2-45, 13-1–13-47 angle between two, 2-11–2-12 concurrent straight lines, 2-10–2-11 condition of a general equation of a second degree to represent a pair of, 3-5–3-9 determination of general equation of, 2-1–2-2 equation in symmetrical form, 13-1–13-3 equation passing through two given points, 13-3–13-4 equations of bisectors of angle between, 2-12 equations of two non-intersecting lines, 13-13–13-14 examples related to, 2-12–2-45, 13-17–13-47 intercept form, 2-4–2-5 intersection of three planes, 13-14–13-16

Z01_ANALYTICALGEOMETRY_3604_INDEX.indd 4

intersection of two, 2-8–2-10 normal form, 2-5–2-6 pair of, 3-1–3-35 parametric form and distance form, 2-6–2-7 perpendicular distance on, 2-7–2-8 slope of, 2-3 slope-intercept form of, 2-3–2-4 slope-point form, 2-5 two points form, 2-5 T tangent plane, 15-23–15-26 three dimension, 11-1–11-25 tracing of curves, 10-1–10-15 transverse axis, 8-3 triangle, 1-5–16 triangular prism, 13-16–13-17 two circles, 5-1–5-2 radical axis of, 5-2 two points form, 2-5 V vertex of parabola, 6-3 X XY-term, 10-3

4/23/2013 5:10:21 PM